Add the PCH file name to the message about not being able to read the PCH.

Also add a couple of unit tests to check the invalid-PCH error messages
to satisfy PR4568 and for the assertion (introduced in r149918 and fixed
in r158769) that would cause clang to crash when given an empty PCH.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158772 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed