Dep graph parallel frontend UI test failures

[zetanumbers]

Part of #154314

There are a number of tests for the -Zquery-dep-graph option. Some of them fail with --parallel-frontend-threads set. That's because of this line:

rust/compiler/rustc_middle/src/dep_graph/serialized.rs

Lines 639 to 642 in 06293ff

	// Do not ICE when a query is called from within `with_query`.
	if let Some(retained_graph) = &mut retained_graph.try_lock() {
	retained_graph.push(index, *node, &edges);
	}

Mutex::try_lock might not succeed in parallel and currently such failure is ignored, leaving the retained graph incomplete.

More at #157352

Affected UI tests:

• tests/ui/dep-graph/dep-graph-assoc-type-codegen.rs
• tests/ui/dep-graph/dep-graph-caller-callee.rs
• tests/ui/dep-graph/dep-graph-struct-signature.rs
• tests/ui/dep-graph/dep-graph-trait-impl-two-traits-same-method.rs
• tests/ui/dep-graph/dep-graph-trait-impl.rs
• tests/ui/dep-graph/dep-graph-type-alias.rs
• tests/ui/dep-graph/dep-graph-variance-alias.rs