Track the file kind on SourceLocation rather than computing it from the file name. (#2999)

This will be more correct if a user file is named prelude.carbon, and
reduces the explorer runtime by about 10% by removing a string
comparison from the check for whether trace output is enabled.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

explorer/interpreter/exec_program.cpp

@@ -34,7 +34,7 @@ auto AnalyzeProgram(Nonnull<Arena*> arena, AST ast,
     }
   }
 
-  SourceLocation source_loc("<Main()>", 0);
+  SourceLocation source_loc("<Main()>", 0, FileKind::Main);
   ast.main_call = arena->New<CallExpression>(
       source_loc, arena->New<IdentifierExpression>(source_loc, "Main"),
       arena->New<TupleLiteral>(source_loc));