Don't build and install libcxx when generating a macOS toolchain

[edymtt]

Libcxx is already part of the macOS SDKs shipped since Xcode 12.5, so there is no need for us to rebuild it.
This will also prevent compilation issues in the presets that builds the standard library using a prebuilt toolchain, so the clang compiler that ships in that toolchain does not pickup libcxx headers that do not match the tbd shipped in the underlying macOS SDK.

This change would be desirable for all macOS presets, focusing for now on the toolchain presets.

Addresses rdar://106817494

[edymtt]

@swift-ci please test

[edymtt]

@swift-ci please build toolchain

[compnerd]

I don't have any objections to the removal, but I am concerned that this might break the build. The LLDB builds and tests require that libc++ is built in order to test. Does this not impact that?

[edymtt]

From what I can tell, it should not:

• the macOS LLDB bots are invoking build-script directly and not specifying libcxx, e.g. from logs in https://ci.swift.org/view/LLDB/job/oss-lldb-incremental-macos-cmake/ I can see this invocation

.../build-script --lldb --release --test -- --skip-build-benchmarks --no-swift-stdlib-assertions --lldb-use-system-debugserver --lldb-build-with-cmake --skip-test-swift --skip-test-cmark --cmake-c-launcher /usr/local/bin/sccache --cmake-cxx-launcher /usr/local/bin/sccache

That suggests to me that they already pulling libcxx content from the underlying macOS SDK

• the change is only limited to the toolchain presets, whose products are used in a couple jobs in CI that build the stdlib with a prebuilt compiler

[compnerd]

[etcwilde]

macOS failure:

macos-node-i-047f115b1a70a615a was marked offline: Connection was broken: java.io.EOFException

[edymtt]

@swift-ci please test macOS

[edymtt]

Hit unrelated issue when running swiftpm tests

/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.0.sdk/usr/lib/swift/Foundation.swiftmodule/x86_64-apple-macos.swiftinterface:4:8: error: no such module 'Combine'
import Combine
       ^

[edymtt]

@swift-ci please test macOS

[edymtt]

Hitting another unrelated issue in https://ci.swift.org/job/swift-PR-macos/7344

/Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/swift/SwiftCompilerSources/Sources/SIL/Function.swift:83:41: error: value of type 'BridgedFunction' has no member 'getSILResultType'
  public var resultType: Type { bridged.getSILResultType().type }
                                ~~~~~~~ ^~~~~~~~~~~~~~~~
/Users/ec2-user/jenkins/workspace/swift-PR-macos/branch-main/swift/include/swift/SIL/SILBridging.h:218:18: note: function uses foreign reference type 'SILType' as a value in the return types which breaks 'import_reference' contract (outlined in C++ Interop User Manual).
  swift::SILType getSILResultType() const {
                 ^
Assertion failed: (methodSemantics == CxxRecordSemanticsKind::UnsafePointerMember), function diagnoseUnsafeCxxMethod, file CSDiagnostics.cpp, line 3958.

[edymtt]

@swift-ci please test macOS