[20210107][Frontend] Only compile modules if not already finalized

[bnbarham]

Cherry-pick https://reviews.llvm.org/D105328

---

It was possible to re-add a module to a shared in-memory module cache
when search paths are changed. This can eventually cause a crash if the
original module is referenced after this occurs.

1. Module A depends on B
2. B exists in two paths C and D
3. First run only has C on the search path, finds A and B and loads
   them
4. Second run adds D to the front of the search path. A is loaded and
   contains a reference to the already compiled module from C. But
   searching finds the module from D instead, causing a mismatch
5. B and the modules that depend on it are considered out of date and
   thus rebuilt
6. The recompiled module A is added to the in-memory cache, freeing
   the previously inserted one

This can never occur from a regular clang process, but is very easy to
do through the API - whether through the use of a shared case or just
running multiple compilations from a single CompilerInstance. Update
the compilation to return early if a module is already finalized so that
the pre-condition in the in-memory module cache holds.

Resolves rdar://78180255

Differential Revision: https://reviews.llvm.org/D105328

[bnbarham]

@swift-ci please test