[libclang] Add clang_File_tryGetRealPathName

Summary:
clang_getFileName() may return a path relative to WorkingDir.
On Arch Linux, during clang_indexTranslationUnit(), clang_getFileName() on
CXIdxIncludedIncludedFileInfo::file may return
"/../lib64/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../include/c++/7.3.0/string",
for `#include <string>`.

I presume WorkingDir is somehow changed to /usr/lib or /usr/include and
clang_getFileName() returns a path relative to WorkingDir.

clang_File_tryGetRealPathName() returns "/usr/include/c++/7.3.0/string"
which is more useful for the indexer in this case.

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42893

llvm-svn: 329515