Implement serialization/deserialization of LangOptions in the PCH

Implement serialization/deserialization of LangOptions in the PCH
file. When de-serializing LangOptions, we check that the
currently-provided language options are consistent with the options
used to compile the PCH file. If they are not, we emit a diagnostic
and ignore the PCH file.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@68820 91177308-0d34-0410-b5e6-96231b3b80d8