Manual clang-tidy fixes (#2319)

Note, not trying to address every last error, just some obvious/easy ones.

```
/usr/local/google/home/jperkins/dev/carbon-lang/common/string_helpers.cpp:200:13: warning: prefer transparent functors 'less_equal<>' [modernize-use-transparent-functors]
  auto le = std::less_equal<const char*>();
            ^
/usr/local/google/home/jperkins/dev/carbon-lang/toolchain/semantics/nodes/function.h:22:45: warning: pass by value and use std::move [modernize-pass-by-value]
  Function(ParseTree::Node node, NodeId id, llvm::SmallVector<NodeRef> body)
                                            ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/ast/declaration.cpp:230:14: warning: static member accessed through instance [readability-static-accessed-through-instance]
      return cast<SelfDeclaration>(declaration).name();
             ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/interpreter/type_checker.cpp:1104:33: warning: std::move of the variable 'impl' of the trivially-copyable type 'ConstraintType::ImplConstraint' has no effect [performance-move-const-arg]
    impl_constraints_.push_back(std::move(impl));
                                ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/interpreter/type_checker.cpp:1145:36: warning: std::move of the variable 'rewrite' of the trivially-copyable type 'ConstraintType::RewriteConstraint' has no effect [performance-move-const-arg]
    rewrite_constraints_.push_back(std::move(rewrite));
                                   ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/interpreter/type_checker.cpp:1156:32: warning: std::move of the variable 'context' of the trivially-copyable type 'ConstraintType::LookupContext' has no effect [performance-move-const-arg]
    lookup_contexts_.push_back(std::move(context));
                               ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/fuzzing/fuzzverter.cpp:74:42: warning: argument name 'trace' in comment does not match parameter name 'parser_debug' [bugprone-argument-comment]
                                         /*trace=*/false);
                                         ^
./explorer/syntax/parse.h:19:17: note: 'parser_debug' declared here
           bool parser_debug) -> ErrorOr<Carbon::AST>;
                ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/fuzzing/proto_to_carbon_test.cpp:34:55: warning: argument name 'trace' in comment does not match parameter name 'parser_debug' [bugprone-argument-comment]
    const ErrorOr<AST> ast = Carbon::Parse(&arena, f, /*trace=*/false);
                                                      ^
./explorer/syntax/parse.h:19:17: note: 'parser_debug' declared here
           bool parser_debug) -> ErrorOr<Carbon::AST>;
                ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/fuzzing/proto_to_carbon_test.cpp:42:41: warning: argument name 'trace' in comment does not match parameter name 'parser_debug' [bugprone-argument-comment]
          &arena, f, source_from_proto, /*trace=*/false);
                                        ^
./explorer/syntax/parse.h:25:59: note: 'parser_debug' declared here
                     std::string_view file_contents, bool parser_debug)
                                                          ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/syntax/parse_test.cpp:27:60: warning: argument name 'trace' in comment does not match parameter name 'parser_debug' [bugprone-argument-comment]
      ParseFromString(&arena, "file.carbon", FileContents, /*trace=*/false);
                                                           ^
./explorer/syntax/parse.h:25:59: note: 'parser_debug' declared here
                     std::string_view file_contents, bool parser_debug)
                                                          ^
/usr/local/google/home/jperkins/dev/carbon-lang/migrate_cpp/cpp_refactoring/var_decl.cpp:57:58: warning: string concatenation results in allocation of unnecessary temporary strings; consider using 'operator+=' or 'string::append()' instead [performance-inefficient-string-concatenation]
      segments.push_back({type_loc_class, qual_str + " " + range_str});
                                                         ^
/usr/local/google/home/jperkins/dev/carbon-lang/explorer/fuzzing/ast_to_proto_test.cpp:105:55: warning: argument name 'trace' in comment does not match parameter name 'parser_debug' [bugprone-argument-comment]
    const ErrorOr<AST> ast = Carbon::Parse(&arena, f, /*trace=*/false);
                                                      ^
./explorer/syntax/parse.h:19:17: note: 'parser_debug' declared here
           bool parser_debug) -> ErrorOr<Carbon::AST>;
                ^
```

migrate_cpp/cpp_refactoring/var_decl.cpp

@@ -5,6 +5,7 @@
 #include "migrate_cpp/cpp_refactoring/var_decl.h"
 
 #include "clang/ASTMatchers/ASTMatchers.h"
+#include "llvm/Support/FormatVariadic.h"
 
 namespace cam = ::clang::ast_matchers;
 
@@ -54,7 +55,8 @@ auto VarDecl::GetTypeStr(const clang::VarDecl& decl) -> std::string {
     } else if (range_str.empty()) {
       segments.push_back({type_loc_class, qual_str});
     } else {
-      segments.push_back({type_loc_class, qual_str + " " + range_str});
+      segments.push_back(
+          {type_loc_class, llvm::formatv("{0} {1}", qual_str, range_str)});
     }
 
     type_loc = type_loc.getNextTypeLoc();