fix: support std.compat with Clang and MSVC STL #768

Reproduction

On Windows, configure a C++23 executable with the LLVM toolchain and an MSVC sysroot:

[package]
name = "std-compat-canary"
version = "0.1.0"
standard = "c++23"

[toolchain]
windows = "llvm@22.1.8"

[target.x86_64-windows-msvc]
sysroot = "msvc@system"

[build]
cxx_runtime = "host-coupled"

[targets.std-compat-canary]
kind = "bin"
main = "src/main.cpp"

import std.compat;

int main() {
    auto* memory = ::malloc(32);
    if (!memory) return 1;
    ::free(memory);
    std::vector<int> values{1, 2, 3};
    ::printf("compat=%zu\n", values.size());
    return values.size() == 3 ? 0 : 1;
}

Run mcpp build, then run the executable.

Actual behavior

Compilation fails with module 'std.compat' not found, although the selected MSVC toolset provides both std.ixx and std.compat.ixx.

bind_msvc_sysroot() deliberately binds only std.ixx. Its comment records another underlying problem: the Clang compat builder does not specify -x c++-module, so Clang treats .ixx as linker input and produces no BMI. The ordinary std builder already handles this extension.

Expected behavior

Both modules should come from the selected MSVC toolset, and Clang should parse std.compat.ixx as a module interface. Native MSVC and Clang/libc++ paths should retain their existing behavior.

Environment and verification

• Windows x64
• mcpp 2026.10.3.1; source baseline 6e3cb974d199546112ef13f65597a912a06cb899
• LLVM 22.1.8, using clang++ with MSVC STL
• Reproduced with MSVC 14.44.35207
• A local fix builds and runs the canary with both MSVC 14.44.35207 and 14.51.36231 sysroots, printing compat=3
• An unmodified native MSVC build also succeeds