Support Windows build compilers to match LLVM as closely as possible.

[RichBarton-Arm]

LLVM supports being built on Windows - see https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

We need to try and claim the same support, including for the compilers - C++17 requirements notwithstanding.

[isuruf]

List of related issues and PRs

• [x] https://github.com/flang-compiler/f18/issues/1034
• [x] https://github.com/flang-compiler/f18/issues/1007
• [ ] https://github.com/flang-compiler/f18/pull/1032
• [x] https://github.com/flang-compiler/f18/pull/1029
• [x] https://github.com/flang-compiler/f18/pull/1028
• [ ] https://github.com/flang-compiler/f18/pull/1025
• [ ] https://github.com/flang-compiler/f18/pull/1008
• [x] https://github.com/flang-compiler/f18/pull/1006
• [x] https://github.com/flang-compiler/f18/pull/1036