Support interpolated block for try and async

[compiler-errors]

I'm putting this up for T-lang discussion, to decide whether or not they feel like this should be supported. This was raised in #112952, which surprised me. There doesn't seem to be a technical reason why we don't support this.

Precedent:

This is supported:

macro_rules! always {($block:block) => {iftrue $block
}}fnmain(){always!({});}

Counterpoint:

However, for context, this is not supported:

macro_rules! unsafe_block {($block:block) => {unsafe $block
}}fnmain(){unsafe_block!({});}

If this support for async and try with interpolated blocks is not desirable, then I can convert them to instead the same diagnostic as unsafe $block and make this situation a lot less ambiguous.

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I'll try to write up more before T-lang triage on Tuesday. I couldn't find anything other than #69760 for why something like unsafe $block is not supported, and even that PR doesn't have much information.

Fixes#112952