Revert "fix bug that undefined internal is a warning only for -pedantic-errors (llvm#98016)"

This reverts commit e16882f.

Broken bots:
https://lab.llvm.org/buildbot/#/builders/144/builds/2080

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -592,9 +592,6 @@ Attribute Changes in Clang
 
 Improvements to Clang's diagnostics
 -----------------------------------
-- Clang now emits an error instead of a warning for ``-Wundefined-internal``
-  when compiling with `-pedantic-errors` to conform to the C standard
-
 - Clang now applies syntax highlighting to the code snippets it
   prints.
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6016,7 +6016,7 @@ def note_deleted_assign_field : Note<
   "because field %2 is of %select{reference|const-qualified}4 type %3">;
 
 // These should be errors.
-def warn_undefined_internal : ExtWarn<
+def warn_undefined_internal : Warning<
   "%select{function|variable}0 %q1 has internal linkage but is not defined">,
   InGroup<DiagGroup<"undefined-internal">>;
 def err_undefined_internal_type : Error<