[libclang] Introduce libclang APIs for creating a buffer with a JSON virtual file overlay description.

The current API only supports adding 'virtual file path' -> 'real file path' mappings.

rdar://15986708

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202105 91177308-0d34-0410-b5e6-96231b3b80d8