Improve MSVC debug info for f16 and f128

[tgross35]

The code at

rust/compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

Lines 696 to 707 in 6cbf092

	impl MsvcBasicName for ty::FloatTy {
	fn msvc_basic_name(self) -> &'static str {
	// FIXME: f16 and f128 have no MSVE representation. We could improve the debuginfo.
	// See: <https://github.com/rust-lang/rust/pull/114607/files#r1454683264>
	match self {
	ty::FloatTy::F16 => "half",
	ty::FloatTy::F32 => "float",
	ty::FloatTy::F64 => "double",
	ty::FloatTy::F128 => "fp128",
	}
	}
	}

produces a debug representation for MSVC, which could be improved. Quoting from @MaulingMonkey in https://github.com/rust-lang/rust/pull/114607/files#r1454683264:

Not sure if proper support is possible here without bugging Microsoft. Even if it is possible, Microsoft hasn't bothered for their own SDK types (e.g. _FLOAT128 can be found in the windows SDK, but it's just a struct around a couple of u64s and has no useful debug visualizers.)

If it were possible:

* Emit debug info for a `u16`/`u128`-wrapping structure (I'm fuzzy on the details here, I've only wrapped existing emitted types rather than emitting my own.  Merely defining an unused std type might not be enough as such types will get culled IME?)

* Write/extend a `*.natvis` file (probably [rust/src/etc/natvis/intrinsics.natvis](https://github.com/rust-lang/rust/blob/6bf600bc98879bf1bf2ab0f3b3b37c965f5bdff6/src/etc/natvis/intrinsic.natvis)?) that unpacks the bits and reconstitutes them as an approximate double or float in it's `<DisplayString>`.
  
  * How do you transmute/type-pun from `u64` → `f64` in the subset of C++ allowed by the debugger (no pointer-to-temporary shenannigans)?  _Maybe_ abuse a union somehow...?
  * See [`natvis-pdbs` Recommended Reading](https://crates.io/crates/natvis-pdbs#recommended-reading)
  * If you added a new `*.nativs` file for some reason, make sure it's referenced properly (see e.g. [rust/src/bootstrap/src/core/build_steps/dist.rs](https://github.com/rust-lang/rust/blob/master/src/bootstrap/src/core/build_steps/dist.rs#L535-L538)
  * Consider unit tests like [tests/debuginfo/pretty-std-collections-hash.rs](https://github.com/rust-lang/rust/blob/6bf600bc98879bf1bf2ab0f3b3b37c965f5bdff6/tests/debuginfo/pretty-std-collections-hash.rs#L1)

(It's possible to call _CrtCheckMemory() from a watch window. I forget if this works in a *.natvis visualizer. If so, you could theoretically author F128toApproxF64(high: u64, low: u64) and stuff it in a DLL? Or better yet, let someone more foolish take the time to go down that rabbit hole and shave that yak...)

@rustbot label +F-f16_and_f128

[tgross35]

@rustbot label +A-debuginfo +O-windows-msvc +T-compiler -needs-triage +E-easy

[ismailarilik]

Or better yet, let someone more foolish take the time to go down that rabbit hole and shave that yak...

Well, how can that be easy? =)

I saw a few issues which seems not easy. I think we should define what is "easy".