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/syntax/parse_test.cpp

@@ -23,8 +23,9 @@ fn Foo() {}
 
 TEST(ParseTest, ParseFromString) {
   Arena arena;
-  ErrorOr<AST> parse_result = ParseFromString(
-      &arena, "file.carbon", FileContents, /*parser_debug=*/false);
+  ErrorOr<AST> parse_result =
+      ParseFromString(&arena, "file.carbon", FileKind::Main, FileContents,
+                      /*parser_debug=*/false);
   ASSERT_TRUE(parse_result.ok());
   EXPECT_EQ(parse_result->declarations.size(), 1);
 }