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/ast/pattern_test.cpp

@@ -24,7 +24,7 @@ using testing::IsEmpty;
 MATCHER(AutoField, "") { return isa<AutoPattern>(*arg); }
 
 static auto FakeSourceLoc(int line_num) -> SourceLocation {
-  return SourceLocation("<test>", line_num);
+  return SourceLocation("<test>", line_num, FileKind::Main);
 }
 
 class PatternTest : public ::testing::Test {