When the ASTImporter imports a source location, it avoids importing macro

When the ASTImporter imports a source location, it avoids importing macro
expansions by calling getSpellingLoc(). That's great in most cases, but for
macros defined in the '<built-in>' source file, the source file is invalid
and does not import correctly, causing an assertion failure (the assertion
is Invalid SLocOffset or bad function choice).

A more reliable way to avoid this is to use getFileLoc(), which does not
return built-in locations. This avoids the crash but still preserves valid
source locations.

I've added a testcase that covers the previously crashing scenario.

https://reviews.llvm.org/D26054

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286144 91177308-0d34-0410-b5e6-96231b3b80d8