Frontend: Refactor compileModuleAndReadAST, NFC

This renames `compileModuleAndReadAST`, adding a `BehindLock` suffix,
and refactors it to significantly reduce nesting.

- Split out helpers `compileModuleAndReadASTImpl` and
  `readASTAfterCompileModule` which have straight-line code that doesn't
  worry about locks.
- Use `break` in the interesting cases of `switch` statements to reduce
  nesting.
- Use early `return`s to reduce nesting.

Detangling the compile-and-read logic from the check-for-locks logic
should be a net win for readability, although I also have a side
motivation of making the locks optional in a follow-up.

No functionality change here.

Differential Revision: https://reviews.llvm.org/D95581