cmd/link: support LLVM/Clang for MSVC

[kkqy]

I find that cgo can compatible with LLVM/Clang for MSVC well actually.
The only problem I have found is that some arguments for linker of Mingw/GCC can't be used in linker of MSVC, so I write a judgement.
If go detect that the toolchain is LLVM/Clang for MSVC, do not pass those arguments that are not supported by linker of MSVC.

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[kkqy]

Updates #42146
Updates #49080

[kkqy]

I find the second problem today that is the program using Clang for MSVC can't be debugged in Go.
I have no idea to fix it because it seems like Golang supports debug info of GCC only.
so this commit only help us to link our golang program to those libraries of built-by-MSVC and closed source.

Mingw/GCC has defferent C++ ABI from MSVC,so we can't use C++ library of MSVC in Mingw/GCC,
but there are so many commercial libraries that are closed source and built by MSVC.
It is regretful if Golang has no easy solution to be compatible with them.

We can build at least now....hm....

[seankhliao]

please use Gerrit for discussing the change, we don't use GitHub pull requests for this

[gopherbot]

Message from 林志豪:

Patch Set 1:

(1 comment)

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[gatspy]

how use msvc complie cgo?

[ianlancetaylor]

@gatspy Please see https://go.dev/wiki/Questions.

[kkqy]

@gatspy you need:

1. Edit the source like this commit, and remove those flags that are available only in mingw.
2. Then install the Clang for MSVC (which use MSVC's compiler as its backend), and add the Clang into the PATH.
3. set environment：

CC="clang"
CXX="clang++"

now, your code in CGO can be compiled by compiler of MSVC.