libclang: add clang_getTypeSpelling(CXType CT)

Adds a function clang_getTypeSpelling(CXType CT) that returns
a CXString containing the underlying type.

Patch by Ben Gertzfield.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175299 91177308-0d34-0410-b5e6-96231b3b80d8