check each of the files within that directory to determine if any of
them is an AST file that matches the language and target options. If
so, the first matching AST file is loaded. This fixes a longstanding
discrepency with GCC's precompiled header implementation.
llvm-svn: 166469
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.
llvm-svn: 158772