[Serialization] A temporary fix for LocatableType

Gracefully handle `LocatableType` types if they show up during
serialization. This is a temporary fix until we can remove
`TransitivelyConformsTo` constraint from the solver which is
the underlying cause of the issue (see #82541).

Resolves: rdar://153461854
(cherry picked from commit 907606e)

Author: xedin

lib/Serialization/Serialization.cpp

@@ -5601,8 +5601,8 @@ class Serializer::TypeSerializer : public TypeVisitor<TypeSerializer> {
     llvm_unreachable("error union types do not persist in the AST");
   }
 
-  void visitLocatableType(const LocatableType *) {
-    llvm_unreachable("locatable types do not persist in the AST");
+  void visitLocatableType(const LocatableType *LT) {
+    visit(LT->getSinglyDesugaredType());
   }
 
   void visitBuiltinTypeImpl(Type ty) {

test/Serialization/transitive_conformances_and_cgfloat_double.swift

@@ -0,0 +1,32 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -emit-module %s -o %t
+
+// REQUIRES: objc_interop
+
+// rdar://153461854
+
+import CoreGraphics
+
+@resultBuilder
+public struct Builder {
+  public static func buildBlock<T1>(_ t1: T1) -> (T1) {
+    return (t1)
+  }
+}
+
+protocol P {
+}
+
+struct Proxy: P {
+  let to: (CGFloat?) -> Void
+}
+
+struct Test {
+  @Builder var proxy: some P {
+    Proxy { point in
+      if let point {
+        let _: SIMD2<Double>? = SIMD2(point, point)
+      }
+    }
+  }
+}
+