[5.9][ConstraintSystem] Use an explicit generic argument constraint for UnresolvedSpecializeExpr.

[hborla]

• Explanation: The eager generic argument matching code in CSGen for unresolved specializations was incorrect because it didn't account for parameter packs, which lead to ambiguity error messages when using a variadic generic type that shares a name with a macro. Fortunately, a constraint to delay explicit generic argument matching already exists for macros. Use it here. This also fixes a longstanding issue where references to generic typealiases were handled incorrectly in expressions, leading to bogus and misleading error messages.
• Issue: rdar://110800793
• Risk: Low. Most generic code does not go through the UnresolvedSpecializeExpr code path, and many of the cases that do didn't work properly anyway.
• Testing: Added new tests for the macro/type ambiguity, and updated existing type alias tests that had incorrect behavior (removing some FIXMEs from 2017!)
• Reviewer: @DougGregor
• Main branch PR: [ConstraintSystem] Use an explicit generic argument constraint for UnresolvedSpecializeExpr. #66657

[hborla]

@swift-ci please test