fix(naga): Fix builtin metal imports

[madsmtm]

Connections
Might help with #5827.
Part of #4632.
See also related past issue gfx-rs/naga#1625.

Description
In gfx-rs/naga#289 / gfx-rs/naga#239, using namespace metal; was removed, probably to help with avoiding name clashes. This causes issues on older platforms such as macOS 10.12 though, since a lot of types there doesn't seem to be builtin, and are only exposed as typedefs in the metal namespace. Naga already contains a partial workaround for this (the using metal::uint;).

TODO: Solution?

Testing
cargo xtask test on a macOS 10.12 machine with #8953 applied gets us down to 11 failed tests.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests.
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[madsmtm]

This PR is a draft because I see three ways of resolving this:

1. Re-add using namespace metal;, and add everything from that namespace to the list of reserved keywords.
2. Namespace all non-C types (e.g. do metal::uint, metal::uint2, metal::float4, etc). This would be partially reverting [msl-out] Write 'using metal::uint' at the top, to work around bug. naga#1740.
3. Explicitly import all the basic types that Naga uses (e.g. do using metal::{uint, uint2, uint4, ...};).

CC @kvark who avoided using namespace metal; and implemented the keywords stuff, WDYT?