Teach ASTContext and Preprocessor to hold on to references to the same

LangOptions, rather than making distinct copies of
LangOptions. Granted, LangOptions doesn't actually get modified, but
this will eventually make it easier to construct ASTContext and
Preprocessor before we know all of the LangOptions.




git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138959 91177308-0d34-0410-b5e6-96231b3b80d8