[CLANG] Enable alignas after GNU attributes

Author: DenisGZM

clang/lib/Parse/ParseStmt.cpp

@@ -296,6 +296,11 @@ StmtResult Parser::ParseStatementOrDeclarationAfterAttributes(
     goto Retry;
   }
 
+  case tok::kw_alignas: {
+    ParseAlignmentSpecifier(CXX11Attrs);
+    goto Retry;
+  }
+
   case tok::kw_template: {
     SourceLocation DeclEnd;
     ParseTemplateDeclarationOrSpecialization(DeclaratorContext::Block, DeclEnd,

clang/test/SemaCUDA/cuda-attr-order.cu

@@ -0,0 +1,15 @@
+// Verify that we can parse a simple CUDA file with different attributes order.
+// RUN: %clang_cc1 "-triple" "nvptx-nvidia-cuda"  -fsyntax-only -verify %s
+// expected-no-diagnostics
+#include "Inputs/cuda.h"
+
+struct alignas(16) float4 {
+    float x, y, z, w;
+};
+
+__attribute__((device)) float func() {
+    __shared__ alignas(alignof(float4)) float As[4][4];  // Both combinations
+    alignas(alignof(float4)) __shared__  float Bs[4][4]; // must be legal
+
+    return As[0][0] + Bs[0][0];
+}