[SR-10559] libdispatch fails to build with _FORTIFY_SOURCE=2

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Issue Description:

The Nix Packages collection on Linux routinely builds with the fortify option (-D_FORTIFY_SOURCE=2). Currently, libdispatch fails to build with the following error:

/home/michael/swift/src/swift-corelibs-libdispatch/src/queue.c:47:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        _dispatch_client_assert_fail(
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/michael/swift/src/swift-corelibs-libdispatch/src/internal.h:1056:3: note: expanded from macro '_dispatch_client_assert_fail'
                asprintf(&_msg, "%s" fmt, DISPATCH_ASSERTION_FAILED_MESSAGE, \
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/nix/store/sr4253np2gz2bpha4gn8gqlmiw604155-glibc-2.27-dev/include/bits/stdio2.h:199:3: note: expanded from macro 'asprintf'
  __asprintf_chk (ptr, __USE_FORTIFY_LEVEL - 1, __VA_ARGS__)
  ^~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/michael/swift/src/swift-corelibs-libdispatch/src/queue.c:56:2: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
        _dispatch_client_assert_fail(
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/michael/swift/src/swift-corelibs-libdispatch/src/internal.h:1056:3: note: expanded from macro '_dispatch_client_assert_fail'
                asprintf(&_msg, "%s" fmt, DISPATCH_ASSERTION_FAILED_MESSAGE, \
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/nix/store/sr4253np2gz2bpha4gn8gqlmiw604155-glibc-2.27-dev/include/bits/stdio2.h:199:3: note: expanded from macro 'asprintf'
  __asprintf_chk (ptr, __USE_FORTIFY_LEVEL - 1, __VA_ARGS__)
  ^~~~~~~~~~~~~~  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

Since the fix is very easy, I was hoping it could be considered, even though _FORTIFY_SOURCE may not be the recommended way of building swift.

Start with src/queue.c and the _dispatch_client_assert_fail macro in src/internal.h, then reproduce the Linux build with CFLAGS including -D_FORTIFY_SOURCE=2. Done means libdispatch builds without the -Wunused-result errors shown in the issue.