c++: rename -fmodules-ts to -fmodules

c++: rename -fmodules-ts to -fmodules

The C++ modules support is not targeting the Modules TS, so it doesn't make
much sense to refer to the TS in the option name.  But keep the old spelling
as an undocumented alias for now.

gcc/ChangeLog:

	* doc/invoke.texi: Rename -fmodules-ts to -fmodules.

gcc/c-family/ChangeLog:

	* c.opt: Add -fmodules with same effect as -fmodules-ts.

gcc/cp/ChangeLog:

	* lang-specs.h: Check fmodules* instead of fmodules-ts.