Make our DIFlags match LLVMDIFlags in the LLVM-C API

[Zalathar]

In order to be able to use a mixture of LLVM-C and C++ bindings for debuginfo, our Rust-side DIFlags needs to have the same layout as LLVM-C's LLVMDIFlags, and we also need to be able to convert it to the DIFlags accepted by LLVM's C++ API.

Internally, LLVM converts between the two types with a simple cast. We can't necessarily rely on that always being true, and LLVM doesn't expose a conversion function, so we have two potential options:

• Convert each bit/subvalue individually
• Statically assert that doing a cast is actually fine

As long as both types do remain the same under the hood (which seems likely), the static-assert-and-cast approach is easier and faster. If the static assertions ever start failing against some future version of LLVM, we'll have to switch over to the convert-each-subvalue approach, which is a bit more error-prone.

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Extracted from #134009, though this PR ended up choosing the static-assert-and-cast approach over the convert-each-subvalue approach.

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[Zalathar]

cc @workingjubilee @klensy

I've managed to convince myself that this way is better than what I was doing in #134009.

Comparing all three values in a static assertion helps to guard against copy-paste errors, and we don't need to care about whether the C++ compiler is going to generate good or terrible code for the manual conversion.