If the precompiled header named by "-include" is actually a directory,

If the precompiled header named by "-include" is actually a directory,
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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@166469 91177308-0d34-0410-b5e6-96231b3b80d8