[lldb][swift] Only run Swift Shell tests when Swift support is enabled

[Teemperor]

Disabling Swift support in LLDB doesn't prevent the test suite from running
the Swift tests (which then end up failing instead of being marked as
unsupported). This adds a lit feature for Swift and adds the REQUIRES to all
Swift tests to mark them as unsupported if Swift is disabled.

[Teemperor]

@swift-ci test

[adrian-prantl]

why isn't this conditional on having swift support enabled?
Ah.. this is the code that declares LLDB_ENABLE_SWIFT_SUPPORT? My CMake is not very good.

[Teemperor]

The GitHub diff doesn't show enough context above this change to get the context, but this code (and the one above) is 'canonicalizing' the bool variable so that we can use it in the lit.site.cfg.py.in file (it essentially just turns all the different strings CMake interprets as True or False into 1 and 0 respectively so that we can write the value into the Python file). Without that our config file would be turn the line config.lldb_enable_swift = @LLDB_ENABLE_SWIFT_SUPPORT@ to config.lldb_enable_swift = Off (as Off is a legit 'false' value in CMake) and then Python would ask 'what does Off mean'? With this line the config file instead gets config.lldb_enable_swift = 0 which Python understands.

[Teemperor]

@swift-ci test

[Teemperor]

@swift-ci test

[Teemperor]

(Seems like BreakpointSimple.test had a line number hardcoded so the REQUIRES broke it)

[Teemperor]

@swift-ci test

[Teemperor]

@swift-ci test

[Teemperor]

swift-ci test platform macos

[Teemperor]

@swift-ci test platform macos

[Teemperor]

@swift-ci test platform macos

[Teemperor]

@swift-ci please test

[Teemperor]

@swift-ci test