c++: Setup aliases imported from modules [PR106820]

c++: Setup aliases imported from modules [PR106820]

I wonder if more generally we need to be doing more work when importing
definitions from header units especially to handle all the work that
'make_rtl_for_nonlocal_decl' and 'rest_of_decl_compilation' would have
been performing. But this patch fixes at least one missing step.

	PR c++/106820

gcc/cp/ChangeLog:

	* module.cc (trees_in::decl_value): Assemble alias when needed.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr106820_a.H: New test.
	* g++.dg/modules/pr106820_b.C: New test.

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