[clang] [AMDGPU] Add builtin to test for constant memory (PR #209679)

[Matt Arsenault via cfe-commits]

================
@@ -47,6 +50,95 @@ static Value *emitAMDGPUSBufferLoadBuiltin(CodeGenFunction &CGF,
                                     CGF.EmitScalarExpr(E->getArg(2))});
 }
 
+constexpr char AMDGPUConstantSectionMarker[] =
+    "__clang_amdgpu_constant_section_marker";
+
+static llvm::GlobalVariable *
----------------
arsenm wrote:

If we're going to have something like this, I'd rather have the generalization that the pointer exists in a specific section 

https://github.com/llvm/llvm-project/pull/209679