IWYU pass on toolchain (#2624)

Just opening files in vscode and seeing what clangd flags.

Some edits to ostream.h to stop it from getting flagged (the usage pattern means it's not always obviously used).

common/check_internal.cpp

@@ -4,6 +4,8 @@
 
 #include "common/check_internal.h"
 
+#include "llvm/Support/Signals.h"
+
 namespace Carbon::Internal {
 
 // Prints the buffered message.

common/check_internal.h

@@ -7,8 +7,6 @@
 
 #include <cstdlib>
 
-#include "llvm/Support/ErrorHandling.h"
-#include "llvm/Support/Signals.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace Carbon::Internal {

common/ostream.h

@@ -9,7 +9,8 @@
 
 #include "common/metaprogramming.h"
 #include "llvm/Support/raw_os_ostream.h"
-#include "llvm/Support/raw_ostream.h"
+// Libraries should include this header instead of raw_ostream.
+#include "llvm/Support/raw_ostream.h"  // IWYU pragma: export
 
 namespace Carbon {
 

toolchain/parser/parse_tree_fuzzer.cpp

@@ -6,7 +6,6 @@
 #include <cstdint>
 #include <cstring>
 
-#include "common/check.h"
 #include "llvm/ADT/StringRef.h"
 #include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/diagnostics/null_diagnostics.h"

toolchain/parser/parse_tree_test.cpp

@@ -9,7 +9,6 @@
 
 #include <forward_list>
 
-#include "llvm/Support/FormatVariadic.h"
 #include "toolchain/common/yaml_test_helpers.h"
 #include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/diagnostics/mocks.h"

toolchain/semantics/semantics_node_block_stack.h

@@ -7,7 +7,6 @@
 
 #include <type_traits>
 
-#include "common/check.h"
 #include "llvm/ADT/SmallVector.h"
 #include "toolchain/semantics/semantics_node.h"
 

toolchain/semantics/semantics_parse_tree_handler.cpp

@@ -13,7 +13,6 @@
 #include "toolchain/lexer/token_kind.h"
 #include "toolchain/lexer/tokenized_buffer.h"
 #include "toolchain/parser/parse_node_kind.h"
-#include "toolchain/semantics/semantics_builtin_kind.h"
 #include "toolchain/semantics/semantics_ir.h"
 #include "toolchain/semantics/semantics_node.h"
 #include "toolchain/semantics/semantics_node_block_stack.h"