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).

llvm-svn: 188968
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 6fd3aa1..5e0acc0 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -6288,7 +6288,7 @@
       if (SourceMgr.isInSystemHeader(RBracketLoc)) {
         SourceLocation IndexLoc = SourceMgr.getSpellingLoc(
             IndexExpr->getLocStart());
-        if (SourceMgr.isFromSameFile(RBracketLoc, IndexLoc))
+        if (SourceMgr.isWrittenInSameFile(RBracketLoc, IndexLoc))
           return;
       }
     }