New llbuild preview build incorrectly sets SDKROOT to Deployment SDK, not the "base SDK"

I decided to take the new llbuild preview builder for a spin in Xcode 9.x with a mixed-objective c and swift project but it failed to find certain system headers (os/os.log) that is in the base SDK. I created a small swift test project and sure enough, the new builder incorrectly sets the SDKROOT to the Deployment Target SDK and not the Base SDK. The legacy builder correctly sets SDKROOT to the Base SDK.

CompileSwiftSources normal x86_64 com.apple.xcode.tools.swift.compiler
    cd /Users/blacey/Projects/LoggingTest
    export DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer
    export SDKROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk
    export SWIFTC="Apple Swift version 4.0.3 (swiftlang-900.0.74.1 clang-900.0.39.2)
Target: x86_64-apple-macosx10.9"
    /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc -incremental -module-name LoggingTest -Onone -enforce-exclusivity=checked -DDEBUG -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk -target x86_64-apple-macosx10.10 -g -module-cache-path /Users/blacey/Library/Developer/Xcode/DerivedData/ModuleCache -Xfrontend -serialize-debugging-options -enable-testing -index-store-path /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Index/DataStore -Xfrontend -report-errors-to-debugger -swift-version 4 -I /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Products/Debug -F /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Products/Debug -parse-as-library -c -j8 /Users/blacey/Projects/LoggingTest/LoggingTest/AppDelegate.swift -output-file-map /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/Objects-normal/x86_64/LoggingTest-OutputFileMap.json -parseable-output -serialize-diagnostics -emit-dependencies -emit-module -emit-module-path /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/Objects-normal/x86_64/LoggingTest.swiftmodule -Xcc -I/Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/swift-overrides.hmap -Xcc -iquote -Xcc /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/LoggingTest-generated-files.hmap -Xcc -I/Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/LoggingTest-own-target-headers.hmap -Xcc -I/Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/LoggingTest-all-target-headers.hmap -Xcc -iquote -Xcc /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/LoggingTest-project-headers.hmap -Xcc -I/Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Products/Debug/include -Xcc -I/Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/DerivedSources/x86_64 -Xcc -I/Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/DerivedSources -Xcc -DDEBUG=1 -emit-objc-header -emit-objc-header-path /Users/blacey/Library/Developer/Xcode/DerivedData/LoggingTest-cuitstdxqxisifaujhctsafcrqkq/Build/Intermediates.noindex/LoggingTest.build/Debug/LoggingTest.build/Objects-normal/x86_64/LoggingTest-Swift.h -Xcc -working-directory/Users/blacey/Projects/LoggingTest

<unknown>:0: error: Swift does not support the SDK 'MacOSX10.10.sdk'
Command CompileSwiftSources failed with a nonzero exit code

Does anyone know how to work-around this problem?

Please file a Xcode bug about this with the test project? It will help the bug get looked at.

Done! Bug 36885560. Sample project attached with steps to reproduce.

By the way, I just discovered a workaround for anyone else who stumbles upon this issue. Set the Base SDK to the same value as latest SDK (i.e. macOS 10.13). Doing so properly sets SDKROOT to the base SDK.