[CodeCompletion] Don't check 'InvalidAsyncContext' for imported globals
'InvalidAsyncContext' depends on the decl context. That may case
"sticky" not-recommended If it's cached for a non-async context.
To workaround this, stop checking 'InvalidAsyncContext' when collecting
completion items for caching. Also consistently use the 'SourceFile' as
the decl context to avoid decl context specific behavior.
rdar://78315441