Update all of the libclang code corresponding to the preprocessor

MacroInstantiation -> MacroExpansion rename. Internally, everything is
switched.

Introduce a new cursor kind enum with the new name, but retain the old
name as an alias so that we don't break backwards compatibility.

Also update the debug printing routine to use 'macro expansions' as its
explicitly not guaranteed to be stable, and mechanically switch the test
cases over to that.

llvm-svn: 135140