Allow some differences between the predefines buffer used to build a

PCH file and the predefines buffer used when including the PCH
file. We (explicitly) detect conflicting macro definitions (rejecting
the PCH file) and about missing macro definitions (they'll be
automatically pulled from the PCH file anyway).

We're missing some checking to make sure that new macro definitions
won't have any impact on the PCH file itself (e.g., #define'ing an
identifier that the PCH file used).

llvm-svn: 70316