[MLIR][GPU] Reject nested symbol references in gpu-kernel-outlining (#188994)

Nested symbol references (e.g. `@module::@func`) inside a `gpu.launch`
body cannot be resolved after the body is outlined into a new
`gpu.module`. Previously, `createKernelModule` used `getLeafReference()`
to look up each symbol use, which silently skipped nested references
when the leaf name could not be found in the parent symbol table. This
left unresolvable cross-module references in the outlined kernel.

This patch detects nested symbol references whose root exists in the
parent symbol table — meaning the reference was valid before outlining
but will become dangling after it — and emits a diagnostic error.
Phantom references whose root does not exist in the parent are left
as-is, preserving existing behavior for unregistered-op attributes
(regression test from #185357).

The existing `@nested_launch` test was inadvertently testing this broken
behavior (silently producing invalid IR with a dangling
`@nested_launch_kernel::@nested_launch_kernel` reference inside the
outlined outer kernel module); it is updated to expect the new error.

Fixes #187942

Assisted-by: Claude Code