Clean up remaining executable_semantics clang-tidy warnings (#906)

Co-authored-by: Geoff Romer <gromer@google.com>

executable_semantics/ast/source_location.h

@@ -26,7 +26,7 @@ class SourceLocation {
   auto operator=(const SourceLocation&) -> SourceLocation& = default;
   auto operator=(SourceLocation&&) -> SourceLocation& = default;
 
-  bool operator==(SourceLocation other) const {
+  auto operator==(SourceLocation other) const -> bool {
     return filename_ == other.filename_ && line_num_ == other.line_num_;
   }