Split isFromMainFile into two functions.

Basically, isInMainFile considers line markers, and isWrittenInMainFile
doesn't.  Distinguishing between the two is useful when dealing with
files which are preprocessed files or rewritten with -frewrite-includes
(so we don't, for example, print useless warnings).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188968 91177308-0d34-0410-b5e6-96231b3b80d8