[ASTUnit] Unlike LoadFromCommandLine, LoadFromCompilerInvocation causes a crash if Precompilepreamble
is set to true because there is no FileManager at that point.

Patch by Hurcan Solter!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173071 91177308-0d34-0410-b5e6-96231b3b80d8