macOS 'swift-latest' alias not updating with new toolchain install

[ematejska]

Imported from JIRA https://bugs.swift.org/browse/TF-553

Type:	Bug	Priority:	Minor
Reporter:	Michael Kowalski	Assignee:	Unassigned
Resolution:	Unresolved	Votes:	0
Labels:	None
Environment:	macOS.  I have only seen the problem updating to the June 1 and now the June 4 Development snapshots.

The macOS alias file at /Library/Developer/Toolchains/swift-latest is not updated when installing a new pre-built toolchain.

For example, if I remove all the files from /Library/Developer/Toolchains/ and then download and install Xcode 10 - 0.3.1 - April 30, 2019 when I run 'swift' in a terminal it shows:

Welcome to Swift version 5.0-dev (LLVM dcb9eb74a7, Clang 95cdf7c9af, Swift 256c9fc69b)
```.
If I then download and install the Development Snapshot Xcode 10 - June 4, 2019 and again run 'swift' it still shows:

Welcome to Swift version 5.0-dev (LLVM dcb9eb74a7, Clang 95cdf7c9af, Swift 256c9fc69b).

I can fix this by either deleting the 'swift-latest' alias before installing a new toolchain or by removing the alias and creating a symbolic link like this:

sudo rm /Library/Developer/Toolchains/swift-latest
sudo ln -s /Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2019-06-04-a.xctoolchain /Library/Developer/Toolchains/swift-latest

Running 'swift' then uses the expected newly installed toolchain.

Welcome to Swift version 5.1-dev (LLVM 082dec2e22, Swift fc896ca).