[Frontend] shrink in-memory PCH buffers to fit

After building a PCH, the vector capacity is on average ~1/3 unused.
If we're going to keep it in memory for a while, reallocate to the right size.
Take care to do this once clang is destroyed so that we can reuse its
memory rather than requesting more.

Differential Revision: https://reviews.llvm.org/D124242