A toy for Doug: Add some 'lit' tests which just run clang++ -fsyntax-only over...

A toy for Doug: Add some 'lit' tests which just run clang++ -fsyntax-only over all the files in a directory.
 - 'lit utils/C++Tests' to run, you may need to fix the stdc++ include path, and your mileage may vary.

 - A handful of the LLVM headers are XFAIL because they don't include the right headers.

llvm-svn: 81990