Split isFromMainFile into two functions.

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