[GSB] Don't add invalid concrete requirements.
When a concrete requirement is invalid due to the concrete type
lacking a conformance to a particular, required protocol, don't emit
that incorrect requirement---it causes invalid states further down the
line.
Fixes SR-5014 / rdar://problem/32402482.
While here, fix a comment that Huon noticed trailed off into oblivion.
(cherry picked from commit dd3869739e5c6aca76ebf66ae71421f01ecabc7c)
(cherry picked from commit e1395ea06f91a2522aa9713dd432bde4904c14f3)
