RequirementMachine: Fix crash when type parameter is fixed to an existential

This is a crash-on-invalid -- the user probably meant to write
"T.Value: Collection" rather than "T.Value == Collection".

Fixes rdar://151479861.

Author: slavapestov

lib/AST/RequirementMachine/ConcreteContraction.cpp

@@ -321,6 +321,10 @@ ConcreteContraction::substTypeParameterRec(Type type, Position position) const {
       return std::nullopt;
     }
 
+    auto *nominalDecl = (*substBaseType)->getAnyNominal();
+    if (!nominalDecl || isa<ProtocolDecl>(nominalDecl))
+      return std::nullopt;
+
     // Substitute the base type into the member type.
     auto *dc = typeDecl->getDeclContext();
     auto subMap = (*substBaseType)->getContextSubstitutionMap(dc);

test/Generics/concrete_contraction_existential.swift

@@ -0,0 +1,13 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P1 {
+  associatedtype Value
+}
+
+protocol P2 {
+  typealias A = Int
+}
+
+struct G<T: P1> where T.Value == any Collection, T.Value.Element: P2 {}
+// expected-error@-1 {{cannot access associated type 'Element' from 'any Collection'; use a concrete type or generic parameter base instead}}
+