Teach the PCH validator to check the preprocessor options, especially

Teach the PCH validator to check the preprocessor options, especially
the macros that are #define'd or #undef'd on the command line. This
checking happens much earlier than the current macro-definition
checking and is far cleaner, because it does a direct comparison
rather than a diff of the predefines buffers. Moreover, it allows us
to use the result of this check to skip over PCH files within a
directory that have non-matching -D's or -U's on the command
line. Finally, it improves the diagnostics a bit for mismatches,
fixing <rdar://problem/8612222>.

The old predefines-buffer diff'ing will go away in a subsequent commit.

llvm-svn: 166641