LLVM assertion regression for msvc

[retep998]

While it's been building fine for a while now, an appveyor build for glutin failed with an LLVM assertion for x86_64-pc-windows-gnu on winapi-build. The i686-pc-windows-gnu build, using the same rustc version, was fine though.

https://ci.appveyor.com/project/tomaka/glutin/build/1.0.1287/job/923vn1edivaxvhl6

rustc 1.3.0-nightly (e6a9be10b 2015-07-07)
cargo 0.4.0-nightly (5f4c188 2015-07-07) (built 2015-07-08)

Compiling winapi-build v0.1.0
     Running `rustc C:\Users\appveyor\.cargo\registry\src\github.com-0a35038f75765ae4\winapi-build-0.1.0\src\lib.rs --crate-name build --crate-type lib -g -C metadata=5e403cad040f7862 -C extra-filename=-5e403cad040f7862 --out-dir C:\projects\glutin\target\debug\deps --emit=dep-info,link -L dependency=C:\projects\glutin\target\debug\deps -L dependency=C:\projects\glutin\target\debug\deps -Awarnings`
Assertion failed: isa<X>(Val) && "cast<Ty>() argument of incompatible type!", file C:\bot\slave\nightly-dist-rustc-win-msvc-64\build\src\llvm\include\llvm/Support/Casting.h, line 237
Could not compile `winapi-build`.

Upon testing locally, it appears the issue is caused by the -g flag.

[retep998]

cc @alexcrichton