Fix inconsistencies in check_typedef_value and remove dead code

Summary: Three fixes inside check_typedef_value:

• IGET/SGET used add_error() directly instead of returning the error like every other case. This prevented the caller from appending context (e.g., “Calling: Foo, Incorrect parameter: 1”). Changed to return for consistency — error messages now include caller context.
• CHECK_CAST used add_error() on the recursive result instead of returning it. Changed to return directly so the caller can append context.
• The empty-string-in-generated-class check inside CONST_STRING was dead code — should_skip_method already skips generated classes before check_typedef_value is ever called. Removed.

Also cached m_is_anonymous_class in run() to avoid repeated type_class + maybe_anonymous_class calls on every check_typedef_value invocation.

Reviewed By: ssj933

Differential Revision: D101055754

fbshipit-source-id: 6c5667406a5aeb283b1c633a05a595027b74c794