Implement -fno-validate-pch at the -cc1 level, which suppresses most

of the usual consistency checks used to determine when a precompiled
header is incompatible with the translation unit it's being loaded
into.

Enable this option when loading a precompiled preamble, because the
preamble loader will be performing all of this checking itself. Enable
the preamble-based test now that it's working.

This option is also useful for debugging Clang's PCH
(<rdar://problem/7532213>).

llvm-svn: 109475