Windows x64 LLVM assert support + MSVC only for none cross compiler build. (#11946)

Windows x64 LLVM assert support + MSVC only for none cross compiler build. (#11946)

* Add support for LLVM asserts in internal LLVM build on Windows.

* Only build LLVM using MSVC if not cross compiling.

If not cross compiling, building runtime where host/target is Windows, only
build and use LLVM as part of msvc build (only supported configuration),
disabled on gcc build.

Enforce 64-bit LLVM support when building using Visual Studio solution file.

* Disable Windows LLVM Visual Studio build on unsupported configurations.

Automatically disable Windows LLVM build in Visual Studio. This enables
the option to configure using --enable-llvm but still build all runtime
configurations from within Visual Studio. A warning will be issued into
build logs when LLVM support is disabled for a specific build configuration.

* Fix so build-init always have cygconf.h available when needed.