Fix a bug with region-parameterized enums etc where trans considered
them to be non-monomorphic.  Merely having lifetime parameters
is not enough to qualify for that status.  Fixes #5243.
auto merge of #5253 : nikomatsakis/rust/issue-5243, r=graydon

them to be non-monomorphic.  Merely having lifetime parameters
is not enough to qualify for that status.  Fixes #5243.

r?