libsyntax: Disallow struct literals after `if`, `while`, `match`, and
`for...in`.

Closes #14803.

If you used a structure literal after one of these keywords, surround it
in parentheses.

[breaking-change]
auto merge of #14885 : pcwalton/rust/struct-literal-tightening, r=alexcrichton

`for...in`.

Closes #14803.

If you used a structure literal after one of these keywords, surround it
in parentheses.

[breaking-change]

r? @nick29581