fix(nvvm): resolve Windows CI build failures

[Snehal-Reddy]

Summary

1. Broken LLVM Download URL: The hardcoded download URL incorrectly used the uppercase tag LLVM-7.1.0 (which has no release assets), instead of the correct lowercase tag llvm-7.1.0. Because curl didn't verify the HTTP response code, the script was attempting to unpack a GitHub 404 HTML page as a tarball, resulting in an obscure UnexpectedEof / TarError panic.
2. Invalid MSVC Compiler Flags: When the cc crate attempts to compile the LLVM C++ shim (RustWrapper.cpp) using MSVC (cl.exe), it blindly passes all flags returned by llvm-config --cxxflags. Since the prebuilt LLVM was cross-compiled, llvm-config returns GCC-style warning flags (e.g. -Wextra), which causes MSVC to fail with a D8021 : invalid numeric argument '/Wextra' error.

Changes

• Corrected PREBUILT_LLVM_URL_LLVM7 to use the lowercase llvm-7.1.0 tag.
• Added an explicit HTTP response code check to catch download failures immediately. It will now fail with a clear, actionable error message showing the bad URL and response code.
• Added logic to filter out all GCC-style warning flags (-W*) when compiling the C++ shim if the target contains "msvc".

Testing

• cargo build passes
• cargo clippy --workspace passes
• Tested on: Windows / Linux

Notes

This issue manifested as build failures specifically when running cargo build in a clean Windows environment where USE_PREBUILT_LLVM triggers.

[brandonros]

i pulled this into #386 if we want