NestedMacroInstantiations -> NestedMacroExpansions

This is switches all the interfaces points (and most of the commenst
/ local variables I saw on my way through) regarding the
NestedMacroInstantiations bit.

The libclang enums corresponding to this state were renamed, but
a legacy enum was added with the old name, and the same value to keep
existing clients working. I've added a documentation blurb for it, but
let me know if there is a canonical way to document legacy elemenst of
the libclang interface.

No functionality changed here, even in tests.

llvm-svn: 135141