Fix checklocks return position for func literals

[Haihan-Jiang]

Fixes #11203.

What

• Use the enclosing function literal position when an SSA return has no source position.
• Apply that return position consistently to checklocks return diagnostics and forced-failure matching.
• Add a regression test for an anonymous deferred function whose implicit return leaves a lock held.

Why

Implicit returns from anonymous functions can have token.NoPos in SSA. checklocks then reports diagnostics at -, making it hard to identify the offending defer.

Testing

• git diff --check
• bazel test //tools/checklocks/test:test_nogo --test_output=all
• bazel test //tools/checklocks/...