[c++]: Improve module-decl diagnostics [PR 98327]

[Nathan Sidwell]

The diagnostic for a misplaced module decl was essentially 'computer
says no', which isn't the most helpful.	 This adjusts it to indicate
what would be acceptable.

	gcc/cp/
         * parser.cc (cp_parser_module_declaration): Alter diagnostic
         text to	say where is permissable.
         gcc/testsuite/
         * g++.dg/modulex/mod-decl-1.C: Adjust.
         * g++.dg/modulex/p0713-2.C: Adjust.
         * g++.dg/modulex/p0713-3.C: Adjust.


-- 
Nathan Sidwell
-------------- next part --------------
A non-text attachment was scrubbed...
Name: pr98327.diff
Type: text/x-patch
Size: 3020 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc-patches/attachments/20210105/8a2a5c07/attachment.bin>