[demangler] Fix demangling of 'half'

Demangle 'Dh' as 'half' (as per GCC), and not 'decimal16' (which doesn't
make sense, as there is no IEEE 754 decimal16 format).

The Itanium C++ ABI specification describes 'Dh' as:
> IEEE 754r half-precision floating point (16 bits)

(https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling-builtin)

Reviewed By: ldionne, jyknight

Differential Revision: https://reviews.llvm.org/D103833