Move the rest of the preprocessor terminology from 'instantiate' and

variants to 'expand'. This changed a couple of public APIs, including
one public type "MacroInstantiation" which is now "MacroExpansion". The
rest of the codebase was updated to reflect this, especially the
libclang code. Two of the C++ (and thus easily changed) libclang APIs
were updated as well because they pertained directly to the old
MacroInstantiation class.

No functionality changed.

llvm-svn: 135139