CI for compiling w/ Clang on Windows

[justsmth]

Description of changes:

• Sets up a Windows CI task to build on that platform with Clang 16.
• Fixes a build failure that seems to only occur when building with Clang on Windows:

D:/a/aws-lc/aws-lc/tool/../third_party/jitterentropy/arch/jitterentropy-base-windows.h:64:13: error: unused function 'jent_get_nstime' [-Werror,-Wunused-function]

static void jent_get_nstime(uint64_t *out)

            ^

1 error generated.

Call-outs:

• Although Clang is used to compile C/C++, the build still uses NASM for the assembly.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.