Add Natvis visualiser and debuginfo tests for `f16`

To render `f16`s in debuggers on MSVC targets, this PR changes the compiler to output `f16`s as `struct f16 { bits: u16 }`, and includes a Natvis visualiser that manually converts the `f16`'s bits to a `float` which is can then be displayed by debuggers. `gdb`, `lldb` and `cdb` tests are also included for `f16` .

`f16`/`f128` MSVC debug info issue: #121837
Tracking issue: #116909