[SYCL] Fix a crash with __bulitin_unique_stable_name (#3284)

A placeholder expression inside a unique-stable-name(at this point, an
unresolved lookup expression) was not properly being transformed during
template instantiation.  This patch ensures that it gets instantiated.

Author: Erich Keane

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -1457,6 +1457,11 @@ static ExprResult TransformUniqueStableName(TemplateInstantiator &TI,
         TI.getSema(), Sema::ExpressionEvaluationContext::Unevaluated);
     ExprResult SubExpr = TI.getDerived().TransformExpr(E->getExpr());
 
+    if (SubExpr.isInvalid())
+      return ExprError();
+
+    SubExpr = TI.getSema().CheckPlaceholderExpr(SubExpr.get());
+
     if (SubExpr.isInvalid())
       return ExprError();
 

clang/test/CodeGenSYCL/unique-stable-name-placeholder-crash.cpp

@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -triple spir64-unknown-unknown-sycldevice -internal-isystem %S/Inputs -std=c++17 -sycl-std=2020 -fsycl -fsycl-is-device -disable-llvm-passes -emit-llvm %s -o - | FileCheck %s
+
+#include <sycl.hpp>
+
+using namespace cl::sycl;
+
+struct A {
+  int a = 0;
+  A() = default;
+};
+constexpr A THE_NAME;
+
+template <auto &R> void temp() {}
+template <auto &R> void foo(const char *out) {
+  out = __builtin_unique_stable_name(temp<R>);
+}
+
+int main() {
+  kernel_single_task<class kernel>(
+      []() {
+        const char *c;
+        foo<THE_NAME>(c);
+      });
+}
+
+// Note: the mangling here is actually the 'typeinfo name for void ()'.  That is
+// because the type of temp<R> is actually the function type (which is void()).
+// CHECK: @__builtin_unique_stable_name._Z3fooIL_ZL8THE_NAMEEEvPKc = private unnamed_addr addrspace(1) constant [9 x i8] c"_ZTSFvvE\00", align 1