[libclang] Change clang_VirtualFileOverlay_writeToBuffer to return a malloc'ed buffer.

Returning CXString is not appropriate if we want to switch to a non-string format buffer.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202675 91177308-0d34-0410-b5e6-96231b3b80d8