[DebugInfo] Refactor code for emitting DWARF expressions for FP constants

This patch moves the selection of the style used to emit the numbers
(DW_OP_implicit_value vs. DW_OP_const+DW_OP_stack_value) into
DwarfExpression::addUnsignedConstant. This logic is not FP-specific, and
it will be needed for large integers too.

The refactor also makes DW_OP_implicit_value (DW_OP_stack_value worked
already) be used for floating point constants other than float and
double, so I've added a _Float16 test for it.

Split off from D90916.

Differential Revision: https://reviews.llvm.org/D91058