Really fix <rdar://problem/7312058> by adding a 'displayDiagnostics' option to

clang_createTranslationUnit() and clang_createTranslationUnitFromSourceFile(). The user can now
specify if the diagnostics from Clang are printed to stderr or are silenced completely. We can
obviously evolve this API to be more general in the future.

Note: Added a FIXME since I wasn't certain what was the best way to redirect to something analogous
to '/dev/null' on Windows.

llvm-svn: 84548