[5.1] [code-completion] Disable diagnostics in @functionBuilder bodies

[benlangmuir]

Cherry-pick #25922 to swift-5.1-branch. Reviewed by @xedin and @rintaro

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When performing code-completion inside the body of a @functionBuilder
closure/function, set the flag to suppress diagnostics. This works
around a big performance problem in some complex bodies that do not
typecheck, which is typical during code-completion. A real-world example
with SwitfUI went from ~50 seconds to 0.5. We do not disable diagnostics
in general because the diagnostic paths provide falback types that are
useful to code-completion.

rdar://52356229

[benlangmuir]

@swift-ci please test

[benlangmuir]

@swift-ci please nominate

• Explanation: When performing code-completion inside the body of a @functionBuilder
  closure/function, set the flag to suppress diagnostics. This works
  around a big performance problem in some complex bodies that do not
  typecheck, which is typical during code-completion. A real-world example
  with SwitfUI went from ~50 seconds to 0.5.
• Scope: Code completion inside @functionBuilder closure and function bodies.
• Issue: rdar://52356229
• Risk: Low
• Testing: Performance improvement verified manually in complex SwiftUI examples. No correctness regressions in existing tests.
• Reviewer: @xedin and @rintaro

[benlangmuir]

Ted approved.