c++: signed __int128_t [PR108099]

The code for handling signed + typedef was breaking on __int128_t, because
it isn't a proper typedef: it doesn't have DECL_ORIGINAL_TYPE.

	PR c++/108099

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Handle non-typedef typedef_decl.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/int128-7.C: New test.