[requires-evolution] Conditionally compiled catch clauses

[owenv]

This is based on #27776, so only the last 2 commits are relevant. It's a WIP for now because I broke code completion (again), but everything else is working so I plan on pitching this on the forums shortly.

This change makes the following legal:

do {
  try performTask()
}
#if os(macOS)
catch TaskError.macOSOnlyError {
  macOSRecovery()
}
#endif
catch { 
  crossPlatformRecovery()
}

[owenv]

This is passing the regular test suite locally now. If anyone sees this I'd be interested in seeing how it fares on the source compatibility suite

[theblixguy]

Can you fix the merge conflict? We can run a source compat test after that

[theblixguy]

@swift-ci please test source compatibility

[theblixguy]

@swift-ci please test source compatibility

[owenv]

Hmm, looks like a couple of failures but nothing too bad. Thanks @theblixguy!

[owenv]

Let's see if this works

@swift-ci please test

[owenv]

For my own future reference: swiftlang/swift-syntax#190 and swiftlang/llvm-project#538 contain the LLDB and swift-syntax changes needed to build a working toolchain.

[owenv]

@DougGregor do you mind kicking off a toolchain build with the two PRs I linked in the comment above? I've tested this locally and was able to build a working macOS toolchain.

[xedin]

@swift-ci please build toolchain

[theblixguy]

swiftlang/swift-syntax#190
swiftlang/llvm-project#538

@swift-ci please build toolchain

[xedin]

Ugh, thanks @theblixguy! I totally missed previous comment...

[swift-ci]

macOS Toolchain
Download Toolchain
Git Sha - fe64516

Install command
tar -zxf swift-PR-27905-465-osx.tar.gz --directory ~/

[owenv]

Closing because conditional compilation was dropped from the final proposal