Swift: Command line builds broken on macOS 10.15

Xcode 11 can build Swift binaries targeting macOS 10.15, but when using the command line tools with CMake's Ninja or Makefile generators, linking an executable fails with

<unknown>:0: error: unable to load standard library for target 'x86_64-apple-macosx10.15'

Manually adding the option

-sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk

to the -emit-executable command line avoids the problem.

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