[libcxx] [ci] Add CI configurations for MinGW

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

NOKEYCHECK=True
GitOrigin-RevId: f5ca3ac748af2c88736212d7dbc105d99ac721c1
diff --git a/docs/index.rst b/docs/index.rst
index a150232..6ed811f 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -117,7 +117,7 @@
 macOS 10.9+     i386, x86_64, arm64       Building the shared library itself requires targetting macOS 10.11+
 FreeBSD 10+     i386, x86_64, arm
 Linux           i386, x86_64, arm, arm64
-Windows         x86_64
+Windows         x86_64                    Both MSVC and MinGW style environments
 AIX             powerpc, powerpc64
 =============== ========================= ============================