Misc changes to SourceManager::ContentCache:

- 'Buffer' is now private and must be accessed via 'getBuffer()'.
   This paves the way for lazily mapping in source files on demand.
- Added 'getSize()' (which gets the size of the content without
   necessarily accessing the MemBuffer) and 'getSizeBytesMapped()'.
- Modifed SourceManager to use these new methods.  This reduces the
  number of places that actually access the MemBuffer object for a file
  to those that actually look at the character data.

These changes result in no performance change for -fsyntax-only on Cocoa.h.

llvm-svn: 61782