[HLSL] Allow narrowing in initialization lists (#108035)

HLSL's initialization lists are _extremely_ generous about allowing
conversions. This change demotes the C++11 warning to the legacy warning
when in HLSL mode.

Required for #56067

Author: llvm-beanz

clang/lib/Sema/SemaInit.cpp

@@ -9548,7 +9548,7 @@ static void DiagnoseNarrowingInInitList(Sema &S,
                       unsigned ConstRefDiagID, unsigned WarnDiagID) {
     unsigned DiagID;
     auto &L = S.getLangOpts();
-    if (L.CPlusPlus11 &&
+    if (L.CPlusPlus11 && !L.HLSL &&
         (!L.MicrosoftExt || L.isCompatibleWithMSVC(LangOptions::MSVC2015)))
       DiagID = IsConstRef ? ConstRefDiagID : DefaultDiagID;
     else

clang/test/AST/HLSL/vector-constructors.hlsl

@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -x hlsl -ast-dump -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -ast-dump -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.0-compute -std=hlsl202x -ast-dump -o - %s | FileCheck %s
 
 typedef float float2 __attribute__((ext_vector_type(2)));
 typedef float float3 __attribute__((ext_vector_type(3)));