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/parser/parse_tree.cpp

@@ -35,6 +35,8 @@ auto ParseTree::Parse(TokenizedBuffer& tokens, DiagnosticConsumer& consumer,
   }
 
   while (!context.state_stack().empty()) {
+    // clang warns on unhandled enum values; clang-tidy is incorrect here.
+    // NOLINTNEXTLINE(bugprone-switch-missing-default-case)
     switch (context.state_stack().back().state) {
 #define CARBON_PARSER_STATE(Name) \
   case ParserState::Name:         \