[clang-tidy] Reuse FileID in getLocation

One FileID per warning will increase and overflow NextLocalOffset
when input file is large with many warnings.
Reusing FileID avoids this problem.

This requires changes in getColumnNumber, D31406, rL299681.

Differential Revision: http://reviews.llvm.org/D31406

llvm-svn: 299700

clang-tools-extra/clang-tidy/ClangTidy.cpp

@@ -239,7 +239,7 @@ private:
       return SourceLocation();
 
     const FileEntry *File = SourceMgr.getFileManager().getFile(FilePath);
-    FileID ID = SourceMgr.createFileID(File, SourceLocation(), SrcMgr::C_User);
+    FileID ID = SourceMgr.getOrCreateFileID(File, SrcMgr::C_User);
     return SourceMgr.getLocForStartOfFile(ID).getLocWithOffset(Offset);
   }