[CrashReproducer] Collect PCH included via -include-pch

Collect the necessary input PCH files.

Do not try to validate the AST before copying it out because if the
crash is in this path, we won't be able to collect it. Instead only
check if it's a file containg an AST.

rdar://problem/27913709

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289460 91177308-0d34-0410-b5e6-96231b3b80d8