Combine lifetime parameters when instantiating default methods

When instantiating trait default methods for certain implementation,
`typeck` correctly combined type parameters from trait bound with those
from method bound, but didn't do so for lifetime parameters. Applies
the same logic to lifetime parameters.

Closes #13204
auto merge of #13503 : edwardw/rust/lifetime-ice, r=nikomatsakis

When instantiating trait default methods for certain implementation,
`typeck` correctly combined type parameters from trait bound with those
from method bound, but didn't do so for lifetime parameters. Applies
the same logic to lifetime parameters.

Closes #13204