Make Sema's use of the preprocessor explicit (rather than assume

the prerocessor will be available via ASTContext).
- Removed the public data member "PP" in ASTContext.
- Changed ASTContext's contructor to take TargetInfo/IdentifierTable explicitly.
- Implicitly create an ASTContext in Sema's constructor. This simplifies
the clients job (and makes ASTContext more private).
--As a side effect, added a "PrintStats" hook to Sema.

To support this level of encapsulation, ASTContext is always dynamically
allocated (by Sema). Previously, ASTContext was being allocated on the
stack. I don't believe this should be a performance issue (since ASTContext
is fairly course grain and tied to the lifetime of Sema currently).

llvm-svn: 39345