c++: fix 'unsigned __int128_t' semantics [PR108099]

My earlier patch for 108099 made us accept this non-standard pattern but
messed up the semantics, so that e.g. unsigned __int128_t was not a 128-bit
type.

	PR c++/108099

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Keep typedef_decl for __int128_t.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/int128-8.C: New test.