Sema: Fix crash with SpecializeExpr around something that's not a type

Unfortunately the diagnostic is terrible.

Fixes #74858

Author: slavapestov

lib/Sema/CSSimplify.cpp

@@ -13978,6 +13978,8 @@ ConstraintSystem::simplifyExplicitGenericArgumentsConstraint(
     decl = bound->getDecl();
     for (auto argType : bound->getDirectGenericArgs()) {
       auto *typeVar = argType->getAs<TypeVariableType>();
+      if (!typeVar)
+        return SolutionKind::Error;
       auto *genericParam = typeVar->getImpl().getGenericParameter();
       openedTypes.push_back({genericParam, typeVar});
     }

test/Sema/issue-74858.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift
+
+func invalidSpecializeExpr(_ x: DS<Int>.Type) {
+  let y = x<Int>.self
+  // expected-error@-1 {{type of expression is ambiguous without a type annotation}}
+  // FIXME: Bad diagnostic
+}