[C++20] Make operator{==,!=}s const.

In C++20, the compiler will synthesize a version of the operator
with its arguments reversed to ease commutativity. This reversed
version is ambiguous with the hand-written operator when the
argument is const but `this` isn't.

lib/Frontend/CachedDiagnostics.cpp

@@ -46,7 +46,7 @@ struct SerializedSourceLoc {
   unsigned FileID = 0;
   unsigned Offset = 0;
 
-  bool operator==(const SerializedSourceLoc &RHS) {
+  bool operator==(const SerializedSourceLoc &RHS) const {
     return FileID == RHS.FileID && Offset == RHS.Offset;
   }
 };