[AST] Member lookup in the GSB needs to understand equivalence classes with concrete types.

A constraint like `Parameter == SomethingConcrete` means references to
`Parameter` in that context behave like `SomethingConcrete`, including for name
resolution. Handling the parameter as just a parameter type means that it won't
find any non-protocol nested types (i.e. things other than associated types and
protocol typealiases are invisible).

Fixes rdar://problem/42136457 and SR-8240.

validation-test/compiler_crashers_2_fixed/0165-sr8240.swift

@@ -0,0 +1,14 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Box<Representation> {
+    let value: Representation
+}
+enum Repr {}
+extension Repr {
+    typealias RawEnum = ()
+}
+extension Box where Representation == Repr {
+    init(rawEnumValue: Representation.RawEnum) {
+        fatalError()
+    }
+}

validation-test/compiler_crashers_2_fixed/0166-sr8240-2.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+
+struct Box<Representation, T> {
+    let value: Representation
+}
+enum Repr {}
+extension Repr {
+    typealias RawEnum = ()
+}
+extension Box where Representation == Repr, T == Representation.RawEnum {
+    init(rawEnumValue: Representation.RawEnum) {
+        let _: Int.Type = T.self // expected-error {{cannot convert value of type '().Type' to specified type 'Int.Type'}}
+        fatalError()
+    }
+}