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

llvm-svn: 286144