[CodeCompletion] Fix completion for 'catch' pattern bound values

[rintaro]

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. E.g.

func test() throws {
  do {
    try parse(data)
  } catch ParseError.dataCorrupted(let errorMsg) {
    errorMsg.#^COMPLETE^#
  }
}

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 match any patterns.
To resolve this, always type check the body when type checking catch patterns. Also, pretend do {} throws any Error even without any throwing expressions in the body.
rdar://126699879

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci Please smoke test

[rintaro]

I figured this doesn't cover some cases. I will revise this PR

[rintaro]

@swift-ci Please smoke test

[rintaro]

Decided to always type check the body when type checking catch patterns.