Tidy up lint issues. (#3429)

These are manual fixes; mostly from clang-tidy, some from clangd (which
notes unused includes).

In typed_insts, adding inlline due to misc-definitions-in-headers. Per
discussion, clang-tidy is wrong, but inline silences it.

For parameter name skew in definition versus declaration, I'm just using
the name from the definition.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/context.cpp

@@ -24,6 +24,7 @@ enum class RelativeLocation : int8_t {
 };
 
 // Adapts RelativeLocation for use with formatv.
+// TODO: Investigate for approaches that clangd is happier with.
 static auto operator<<(llvm::raw_ostream& out, RelativeLocation loc)
     -> llvm::raw_ostream& {
   switch (loc) {