libclc: remove __attribute__((assume)) for clspv targets

Instead add a proper attribute in clang, and add convert it to
function metadata to keep the information in the IR.
The goal is to remove the dependency on __attribute__((assume)) that
should have not be there in the first place.

Ref #84934

Author: rjodinchr

clang/include/clang/Basic/Attr.td

@@ -4561,3 +4561,9 @@ def CodeAlign: StmtAttr {
     static constexpr int MaximumAlignment = 4096;
   }];
 }
+
+def ClspvLibclcBuiltin: DeclOrStmtAttr {
+  let Spellings = [Clang<"clspv_libclc_builtin">];
+  let Documentation = [Undocumented];
+  let SimpleHandler = 1;
+}

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -979,6 +979,11 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
     EmitKernelMetadata(FD, Fn);
   }
 
+  if (FD && FD->hasAttr<ClspvLibclcBuiltinAttr>()) {
+    Fn->setMetadata("clspv_libclc_builtin",
+                    llvm::MDNode::get(getLLVMContext(), {}));
+  }
+
   // If we are checking function types, emit a function type signature as
   // prologue data.
   if (FD && SanOpts.has(SanitizerKind::Function)) {

libclc/generic/include/clc/clcfunc.h

@@ -7,8 +7,7 @@
 #if defined(CLC_SPIRV) || defined(CLC_SPIRV64)
 #define _CLC_DEF
 #elif defined(CLC_CLSPV) || defined(CLC_CLSPV64)
-#define _CLC_DEF                                                               \
-  __attribute__((noinline)) __attribute__((assume("clspv_libclc_builtin")))
+#define _CLC_DEF __attribute__((noinline)) __attribute__((clspv_libclc_builtin))
 #else
 #define _CLC_DEF __attribute__((always_inline))
 #endif