[AMDGPU] Support the OpenCL generic addrspace feature by default (#137636)

This feature should be supported on AMDGCN architectures with flat
addressing.

clang/lib/Basic/Targets/AMDGPU.h

@@ -318,6 +318,9 @@ public:
       Opts["__opencl_c_images"] = true;
       Opts["__opencl_c_3d_image_writes"] = true;
       Opts["cl_khr_3d_image_writes"] = true;
+
+      Opts["__opencl_c_generic_address_space"] =
+          GPUKind >= llvm::AMDGPU::GK_GFX700;
     }
   }
 

clang/test/Misc/amdgcn.languageOptsOpenCL.cl

@@ -155,3 +155,11 @@
 #endif
 #pragma OPENCL EXTENSION cl_amd_media_ops2: enable
 
+#if (__OPENCL_C_VERSION__ >= 300)
+#ifndef __opencl_c_generic_address_space
+#error "Missing __opencl_c_generic_address_space define"
+#else
+#error "Incorrect __opencl_c_generic_address_space define"
+#endif
+#pragma OPENCL EXTENSION __opencl_c_generic_address_space: enable
+#endif

clang/test/Misc/r600.languageOptsOpenCL.cl

@@ -146,3 +146,8 @@
 #pragma OPENCL EXTENSION cl_khr_subgroups: enable
 // expected-warning@-1{{unsupported OpenCL extension 'cl_khr_subgroups' - ignoring}}
 
+#ifdef __opencl_c_generic_address_space
+#error "Incorrect __opencl_c_generic_address_space define"
+#endif
+#pragma OPENCL EXTENSION __opencl_c_generic_address_space: enable
+// expected-warning@-1{{OpenCL extension '__opencl_c_generic_address_space' unknown or does not require pragma - ignoring}}