avoid spurious lifetime diagnostic in async generic case

[pbkx]

Fixes #115376.

Issue #115376 reports duplicate lifetime diagnostics for a small async generic example where rustc already emits the real T: 'static error. Additionally, it emits diagnostics requiring the same generic type to outlive a shorter anonymous lifetime

My PR keeps the real static-bound diagnostic and filters same-span weaker type-test diagnostics for the same generic when the failed G: 'static bound already covers them. New regression test verifies that the async example emits only the real E0310 diagnostic. One existing borrowck test was updated for the same diagnostic cleanup

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[folkertdev]

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