[CodeCompletion] Fix completion for 'catch' pattern bound values
Previously code completion for 'catch' pattern bound values didn't work
correctly because code completion type checker fails to type check the
value decl in the pattern.
That was because the body of the 'do' statement is not type checked, so
the thrown error is not determined, then falled backed to the default
'Never', which doesn't matches any patterns.
To resolve this, always type check the body when typechecking 'catch'
patterns. Also, pretends 'do {}' throws 'any Error' even without
any throwing expressions in the body.
rdar://126699879
