Increment SwiftPM tools version to 5.5 and minimum deployment target to 10.15.4 on macOS

[abertelrud]

Increment SwiftPM tools version to 5.5 and minimum deployment target to 10.15.4 on macOS

Motivation:

Allow use of Swift 5.5 features such as automatic Codable synthesis. Minimum deployment target of 10.15.4 allows use of FileHandle improvements such as read(upToCount:).

[abertelrud]

@swift-ci please smoke test

[abertelrud]

@benlangmuir would it be possible also for SourceKit-LSP to require 10.15.4 on macOS? Seems like a small change compared with 10.15, and the reason was to allow the new Swift-friendlier FileHandle APIs (that throw Swift.Errors instead of raising ObjC exceptions).

I can open a SourceKit-LSP PR if that isn't a no-go.

[benlangmuir]

@benlangmuir would it be possible also for SourceKit-LSP to require 10.15.4 on macOS? Seems like a small change compared with 10.15, and the reason was to allow the new Swift-friendlier FileHandle APIs (that throw Swift.Errors instead of raising ObjC exceptions).

No concerns from me, we've generally bumped our deployment target whenever swiftpm decides to, I think. CCing @akyrtzi and @ahoppen in case they want to protest.

[akyrtzi]

We can follow swiftpm on this, I don't have any concern.

[abertelrud]

Great, thanks all for the comments.

[abertelrud]

swiftlang/sourcekit-lsp#436
@swift-ci please smoke test