Manual cleanup of toolchain clang-tidy/clangd warnings. (#3157)

A lot of this is more boring "remove unused header", plus some other
minor cleanups. I think the most significant changes were:

- yaml_test_helpers.cpp is doing a switch on an unsigned int, comparing
to enum values.
-
[bugprone-switch-missing-default-case](https://clang.llvm.org/extra/clang-tidy/checks/bugprone/switch-missing-default-case.html)
is unhappy with EnumBase, but correctly identified
yaml_test_helpers.cpp, so I'm opting to address it rather than disabling
it even though it needs NOLINT in several locations as a result, in
addition to what I think are some low-value `default` cases. I'd be fine
going the other way with this too and disabling it globally (I could see
it being noisier in the explorer).
- MarkInitializerFor swaps the argument names between the .h and .cpp. I
think the .cpp had the order as intended.
- There's a new-ish
[performance-enum-size](https://clang.llvm.org/extra/clang-tidy/checks/performance/enum-size.html)
which I'm basically treating as "add int8_t to enums".

My main motivation here is to just clean up as many of these as I can so
that I stop seeing them in vscode.

toolchain/driver/driver.cpp

@@ -7,12 +7,9 @@
 #include "common/command_line.h"
 #include "common/vlog.h"
 #include "llvm/ADT/ArrayRef.h"
-#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
-#include "llvm/ADT/StringSwitch.h"
 #include "llvm/IR/LLVMContext.h"
-#include "llvm/Support/Format.h"
 #include "llvm/Support/Path.h"
 #include "llvm/TargetParser/Host.h"
 #include "toolchain/codegen/codegen.h"
@@ -43,7 +40,7 @@ can be written to standard output as these phases progress.
 )""",
   };
 
-  enum class Phase {
+  enum class Phase : int8_t {
     Lex,
     Parse,
     Check,
@@ -287,7 +284,7 @@ For questions, issues, or bug reports, please use our GitHub project:
 )""",
   };
 
-  enum class Subcommand {
+  enum class Subcommand : int8_t {
     Compile,
   };