add more detail to the comment describing the bug

Author: adstraw

mlir/lib/Conversion/GPUCommon/GPUOpsLowering.cpp

@@ -543,9 +543,9 @@ LogicalResult GPUPrintfOpToVPrintfLowering::matchAndRewrite(
   // the device code, not the host code
   auto moduleOp = gpuPrintfOp->getParentOfType<gpu::GPUModuleOp>();
 
-  // Convert the location to a valid global location of type FileLineColLoc if
-  // found else UnknownLoc. Remove any metadata from the location which is not
-  // valid for a global location.
+  // Create a valid global location removing any metadata attached to the
+  // location as debug info metadata inside of a function cannot be used outside
+  // of that function.
   Location globalLoc = loc->findInstanceOfOrUnknown<FileLineColLoc>();
 
   auto vprintfType =

mlir/test/Conversion/GPUToNVVM/gpu-to-nvvm-debuginfo.mlir

@@ -13,7 +13,7 @@
 // CHECK-DAG: [[LOC:#[a-zA-Z0-9_]+]] = loc("foo.mlir":0:0)
 #loc = loc("foo.mlir":0:0)
 
-gpu.module @test_module_56 {
+gpu.module @test_module_1 {
   // CHECK-DAG: llvm.mlir.global internal constant @[[$PRINT_GLOBAL0:[A-Za-z0-9_]+]]("Hello, world with location\0A\00") {addr_space = 0 : i32} loc([[LOC]])
   // CHECK-DAG: llvm.func @vprintf(!llvm.ptr, !llvm.ptr) -> i32 loc([[LOC]])