[5.6] Revert "[SourceKit] Don't build an AST if no semantic info is requested"

[ahoppen]

Cherry-pick #40491 to release/5.6.

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Reverts #40353

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The change actually wasn’t correct because we didn’t consider the following scenario:

• The client makes an edit and had key_enablesyntaxmap = key_enablesubstructure = key_enablediagnostics = 0 and
  syntax tree transfer mode set to off
• The request returned immediately without providing semantic information but build an AST in the background
• The client would later retrieve diagnostics using a 0,0 replace text request that had key_enablediagnostics = 1. That request doesn’t cause the AST to rebuild but just returns the latest semantic information.

With the PR that is being reverted we wouldn’t build an AST at all and thus we won’t have any diagnostics to return from the 0,0 replace text request.

rdar://86289587

[ahoppen]

@swift-ci Please smoke test

[ahoppen]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 5260d65

[ahoppen]

@swift-ci Please test macOS