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_test.cpp

@@ -12,13 +12,10 @@
 #include <utility>
 
 #include "llvm/ADT/ScopeExit.h"
-#include "llvm/ADT/SmallString.h"
 #include "llvm/Object/Binary.h"
-#include "llvm/Support/FileSystem.h"
-#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/FormatVariadic.h"
 #include "testing/base/test_raw_ostream.h"
 #include "toolchain/base/yaml_test_helpers.h"
-#include "toolchain/diagnostics/diagnostic_emitter.h"
 
 namespace Carbon::Testing {
 namespace {