Once we've built a precompiled preamble, keep track of the details of

that preamble (the preamble text, preamble file, reserved main file
size). Check these details when we try to rebuild the precompiled
preamble, and when nothing has changed, re-use the precompiled
preamble.

This code is still very much a WIP, and can't even properly be tested
because we have no way to use the precompiled preamble yet. "Trust me"

llvm-svn: 109294