c++/modules: Improve diagnostic when redeclaring builtin in module [PR102345]

c++/modules: Improve diagnostic when redeclaring builtin in module [PR102345]

If a user mistakenly includes a standard library header within the
module purview, they currently get a confusing "declaration conflicts
with builtin" error.  This patch updates the message to include "in
module", to help guide the user towards the likely cause.

	PR c++/102345

gcc/cp/ChangeLog:

	* module.cc (module_may_redeclare): Update error message.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/enum-12.C: Test for updated error.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>