Modules: Cache PCMs in memory and avoid a use-after-free

Clang's internal build system for implicit modules uses lock files to
ensure that after a process writes a PCM it will read the same one back
in (without contention from other -cc1 commands).  Since PCMs are read
from disk repeatedly while invalidating, building, and importing, the
lock is not released quickly.  Furthermore, the LockFileManager is not
robust in every environment.  Other -cc1 commands can stall until
timeout (after about eight minutes).

This commit changes the lock file from being necessary for correctness
to a (possibly dubious) performance hack.  The remaining benefit is to
reduce duplicate work in competing -cc1 commands which depend on the
same module.  Follow-up commits will change the internal build system to
continue after a timeout, and reduce the timeout.  Perhaps we should
reconsider blocking at all.

This also fixes a use-after-free, when one part of a compilation
validates a PCM and starts using it, and another tries to swap out the
PCM for something new.

The PCMCache is a new type called MemoryBufferCache, which saves memory
buffers based on their filename.  Its ownership is shared by the
CompilerInstance and ModuleManager.

  - The ModuleManager stores PCMs there that it loads from disk, never
    touching the disk if the cache is hot.

  - When modules fail to validate, they're removed from the cache.

  - When a CompilerInstance is spawned to build a new module, each
    already-loaded PCM is assumed to be valid, and is frozen to avoid
    the use-after-free.

  - Any newly-built module is written directly to the cache to avoid the
    round-trip to the filesystem, making lock files unnecessary for
    correctness.

Original patch by Manman Ren; most testcases by Adrian Prantl!

llvm-svn: 298165

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