[libclang] Fix crash when a #pragma diagnostic is included in the preamble.

A PCH file keeps track of #pragma diagnostics state; when loading the preamble, they conflicted
with the #pragma diagnostic state already present in the DiagnosticsEngine object due to
parsing the preamble.

Fix this by clearing the state of the DiagnosticsEngine object.
Fixes rdar://10363572 && http://llvm.org/PR11254.

llvm-svn: 143644