cmSourceFileLocation: Avoid string allocation in extension checking

The substr call was causing excess allocations. Swap the cheaper
character check to be before the longer string comparison, now using the
prefix checking function.

Source/cmSourceFileLocation.cxx

@@ -183,8 +183,9 @@ cmSourceFileLocation
   // Check if loc's name could possibly be extended to our name by
   // adding an extension.
   if(!(this->Name.size() > loc.Name.size() &&
-       this->Name.substr(0, loc.Name.size()) == loc.Name &&
-       this->Name[loc.Name.size()] == '.'))
+       this->Name[loc.Name.size()] == '.' &&
+       cmHasLiteralPrefixImpl(this->Name.c_str(),
+                              loc.Name.c_str(), loc.Name.size())))
     {
     return false;
     }