[OpenCL] Fix ICE with invalid use of half

Because half is limited to the `cl_khr_fp16` extension being enabled,
`DefaultLvalueConversion` can fail when it's not enabled.
The original assumption that it will never fail is therefore wrong now.

Fixes: PR47976

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D103175

clang/lib/Sema/SemaExprCXX.cpp

@@ -4193,7 +4193,9 @@ Sema::PerformImplicitConversion(Expr *From, QualType ToType,
   case ICK_Lvalue_To_Rvalue: {
     assert(From->getObjectKind() != OK_ObjCProperty);
     ExprResult FromRes = DefaultLvalueConversion(From);
-    assert(!FromRes.isInvalid() && "Can't perform deduced conversion?!");
+    if (FromRes.isInvalid())
+      return ExprError();
+
     From = FromRes.get();
     FromType = From->getType();
     break;

clang/test/SemaOpenCLCXX/half.clcpp

@@ -0,0 +1,15 @@
+//RUN: %clang_cc1 %s -triple spir -verify -fsyntax-only
+
+#pragma OPENCL EXTENSION cl_khr_fp16 : disable
+
+typedef half half2 __attribute__((ext_vector_type(2)));
+
+half f(half2 h2) { // expected-error{{declaring function return value of type 'half' is not allowed ; did you forget * ?}}
+    return h2.s0; // expected-error{{loading directly from pointer to type '__private half' requires cl_khr_fp16. Use vector data load builtin functions instead}}
+}
+
+#pragma OPENCL EXTENSION cl_khr_fp16 : enable
+
+half f(half2 h2) {
+    return h2.s0;
+}