Introduce CXString type and associated functions clang_getCString() and clang_disposeString().

This abstraction will help us manage string memory for complex names that cross the C++/C boundary (e.g. ObjC methods, selectors). This patch also uses it in clang_getTranslationUnitSpelling (which I'm not sure is necessary). Will investigate later...since the extra malloc() can't hurt (for now).

Patch by John Thompson.

llvm-svn: 86562