[clang][Diagnostic] Simplify emitDiagnosticLoc

[tbaeder]

Summary

We don't use the offset returned from SourceManager::getDecomposedLoc
here, so we might as well just use getFileID().

[aaron.ballman]

LGTM, though please add NFC to the patch summary when landing.

clang/lib/Frontend/TextDiagnostic.cpp

if (LangOpts.MSCompatibilityVersion &&
       OS << ' ';
     OS << ':';
     break;
   }
 
   if (DiagOpts->ShowSourceRanges && !Ranges.empty()) {
     FileID CaretFileID = Loc.getExpansionLoc().getFileID();
     bool PrintedRange = false;
+    const SourceManager &SM = Loc.getManager();
 
     for (const auto &R : Ranges) {
       // Ignore invalid ranges.
       if (!R.isValid())
         continue;
 
-      auto &SM = Loc.getManager();
       SourceLocation B = SM.getExpansionLoc(R.getBegin());
       CharSourceRange ERange = SM.getExpansionRange(R.getEnd());
       SourceLocation E = ERange.getEnd();
-      bool IsTokenRange = ERange.isTokenRange();
-
-      std::pair<FileID, unsigned> BInfo = SM.getDecomposedLoc(B);
-      std::pair<FileID, unsigned> EInfo = SM.getDecomposedLoc(E);
 
-      // If the start or end of the range is in another file, just discard
-      // it.
-      if (BInfo.first != CaretFileID || EInfo.first != CaretFileID)
+      // If the start or end of the range is in another file, just
+      // discard it.
+      if (SM.getFileID(B) != CaretFileID || SM.getFileID(E) != CaretFileID)
         continue;
 
       // Add in the length of the token, so that we cover multi-char
       // tokens.
       unsigned TokSize = 0;
-      if (IsTokenRange)
+      if (ERange.isTokenRange())
         TokSize = Lexer::MeasureTokenLength(E, SM, LangOpts);
 
       FullSourceLoc BF(B, SM), EF(E, SM);
       OS << '{'
          << BF.getLineNumber() << ':' << BF.getColumnNumber() << '-'
          << EF.getLineNumber() << ':' << (EF.getColumnNumber() + TokSize)
          << '}';
       PrintedRange = true;