Push through an easy refactoring to the way we validate and install
implicit constructors. This patch would be NFC but for a regression
test that now must diagnose. #26159 changed validation order in such
a way that the code in validation-test-macosx-x86_64/compiler_crashers_2_fixed/0124-sr5825.swift
used to be accepted. This patch once again changes validation order, so
we now reject this code, restoring the behavior seen on all prior
versions of Swift.
On its face, this test should work. In order for it to do so, witness
matching has to be smarter about the declarations it asks for their
interface type, or it will risk these circular constructions
accidentally being accepted or rejected on a whim.
If unqualified name lookup finds an associated type, but resolution to
the type witness fails, produce a diagnostic rather than silently
failing. Fixes the crash in SR-5825, SR-5881, and SR-5905.
It's conceivable that we could resolve this with suitably global
associated type inference... but that's far off and it's best not to
crash until then.
