This is similar to the existing setting LIBCXX_ABI_DEFINES, with
the difference that this also allows setting other defines than
ones that start with "_LIBCPP_ABI_", and allows setting defines
to a specific value.
This allows avoiding using LIBCXX_TEST_COMPILER_FLAGS in two
CI configurations.
Differential Revision: https://reviews.llvm.org/D116109
Mention support for MinGW in the docs. Rename the existing windows
CI jobs to Clang-cl, as both Clang-cl and MinGW are equally much
"Windows", just different toolchain environments.
Add an XFAIL for a recently added test that fails in the MinGW DLL
configuration (with an explanation of what's causing the failure).
Differential Revision: https://reviews.llvm.org/D112215