Rollup merge of #147004 - estebank:ascription-in-pat, r=fee1-dead

Tweak handling of "struct like start" where a struct isn't supported

This improves the case where someone tries to write a `match` expr where the patterns have type ascription syntax. Makes them less verbose, by giving up on the first encounter in the block, and makes them more accurate by only treating them as a struct literal if successfully parsed as such.

Before, encountering something like `match a { b:` would confuse the parser and think everything after `match` *must* be a struct, and if it wasn't it would generate a cascade of unnecessary diagnostics.

Author: matthiaskrgr