On the parsing side, we treat `a.(b)` as a member access whose second
operand is a `ParenExpr` rather than a `MemberName`. A new node category
is added for the union of `MemberName` and `ParenExpr` to support this.
Checking is mostly reusing the same pieces we already have for simple
member access. Compound member access is in most ways a simplified form
of simple member access because it doesn't need to do any lookup.
This gets to a lifetime subtlety, particularly with things like the
sorting diagnostic consumer that delay output. In order to reduce the
chance of accidental references, disallow StringRef in the diagnostics.
For example:
```
./toolchain/diagnostics/diagnostic_emitter.h:162:5: error: static_assert failed due to requirement '!std::is_same_v<llvm::StringRef, llvm::StringRef>' "Use std::string or llvm::StringLiteral for diagnostic lifetimes."
static_assert(
^
toolchain/check/convert.cpp:477:11: note: in instantiation of member function 'Carbon::Internal::DiagnosticBase<std::string, std::string, llvm::StringRef>::DiagnosticBase' requested here
CARBON_DIAGNOSTIC(StructInitMissingFieldInConversion, Error,
^
./toolchain/diagnostics/diagnostic_emitter.h:47:7: note: expanded from macro 'CARBON_DIAGNOSTIC'
::Carbon::Internal::DiagnosticBase<__VA_ARGS__>( \
^
```
---------
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Continuing with #3070. Just a dir and file rename (mostly removing
prefixes, although for parse_tree_fuzzer and parse_tree_file_test I'm
dropping "tree" instead of "parse"). Everything in the parse dir should
be marked as a move.