asprintf exists on linux

[soloturn]

asprintf exists on linux for quite a while, 2015 at least. do use it.

fixes https://bugs.swift.org/browse/SR-14935

[spevans]

@swift-ci test

[AZero13]

@spevans any updates?

[soloturn]

@gottesmm ?

[etcwilde]

As per the discussion in #4610, __USE_GNU includes asprintf as a C extension and Android's Bionic C library also implements asprintf, so having it defined for these two platforms results in duplicate declarations or duplicate definitions.

So long as -D_GNU_SOURCE is defined, this should build correctly.
The macro is set in CoreFoundation/build.py: https://github.com/apple/swift-corelibs-foundation/blob/cd988935ddf500cab069effe7ccd73aa5cf42fd1/CoreFoundation/build.py#L16-L18
And in CoreFoundation/CMakeLists.txt: https://github.com/apple/swift-corelibs-foundation/blob/cd988935ddf500cab069effe7ccd73aa5cf42fd1/CoreFoundation/CMakeLists.txt#L72-L75

[etcwilde]

@swift-ci please test

[compnerd]

It would be good to verify that _GNU_SOURCE is being defined. I suspect that this might come back to hurt us a bit as not all libc implementation may provide this extension. Perhaps a comment at these sites makes sense to indicate that we are relying on the fact that we only support glibc (and bionic) and that they provide asprintf (at least under some flags).

[drodriguez]

We do for Android in https://github.com/apple/swift/blob/0053526c5d15532ef38fcf72006a9e16840b92a0/lib/ClangImporter/ClangImporter.cpp#L682-L688 . There's a comment for why it is not enabled in Linux, so if you need it for every Linux, maybe it needs to be done manually before any #include that end up in glibc.

[etcwilde]

I'm okay with merging this. We should probably add something like || (TARGET_OS_LINUX && !defined(_GNU_SOURCE)) just to completely cover the bases, but that can come in a subsequent patch.

[etcwilde]

Thanks for the contribution @soloturn!