[AMDGPU] Remove dead function metadata after amdgpu-lower-kernel-arguments

The verifier ensures function !dbg metadata is unique across the module,
so ensure the old nameless function we leave behind doesn't violate
this invariant.

Removing the function via e.g. eraseFromParent seems like a better
option, but doesn't seem to be legal from a FunctionPass.

Author: slinder1

llvm/lib/Target/AMDGPU/AMDGPULowerKernelArguments.cpp

@@ -132,6 +132,7 @@ class PreloadKernelArgInfo {
     NF->setAttributes(AL);
     F.replaceAllUsesWith(NF);
     F.setCallingConv(CallingConv::C);
+    F.clearMetadata();
 
     return NF;
   }

llvm/test/CodeGen/AMDGPU/preload-implicit-kernargs-debug-info.ll

@@ -1,7 +1,7 @@
-; RUN: not --crash opt -mtriple=amdgcn-amd-amdhsa -mcpu=gfx940 -passes='amdgpu-attributor,function(amdgpu-lower-kernel-arguments)' -amdgpu-kernarg-preload-count=16 -S < %s 2>&1 | FileCheck %s
+; RUN: opt -mtriple=amdgcn-amd-amdhsa -mcpu=gfx940 -passes='amdgpu-attributor,function(amdgpu-lower-kernel-arguments)' -amdgpu-kernarg-preload-count=16 -S < %s 2>&1 | FileCheck %s
 
-; CHECK: function declaration may only have a unique !dbg attachment
-; CHECK-NEXT: ptr @0
+; CHECK: define amdgpu_kernel void @preload_block_count_x{{.*}} !dbg ![[#]]
+; CHECK-NOT: declare void @0{{.*}} !dbg ![[#]]
 
 define amdgpu_kernel void @preload_block_count_x(ptr addrspace(1) %out) !dbg !4 !max_work_group_size !7 {
   %imp_arg_ptr = call ptr addrspace(4) @llvm.amdgcn.implicitarg.ptr()