rustc: add lint for parens in if, while, match and return.

The parens in `if (true) {}` are not not necessary, so we'll warn about
them.
Remove unnecessary parentheses.
auto merge of #11663 : huonw/rust/paren-lint, r=cmr

The parens in `if (true) {}` are not necessary, so we'll warn about them.

cc #3070 and #11432