[DebugInfo] Emit negative DW_AT_bit_offset in explicit signed form (#87994)

Before this patch, the value of DW_AT_bit_offset, used for bitfields
before DWARF version 4, was always emitted as an unsigned integer using
the form DW_FORM_data<n>. If the value was originally a signed integer,
for instance in the case of negative offsets, it was up to debug
information consumers to re-cast it to a signed integer.

This is problematic since the burden of deciding if the value should be
read as signed or unsigned was put onto the debug info consumers: the
DWARF specification doesn't define DW_AT_bit_offset's underlying type.
If a debugger decided to interpret this attribute in the form data<n> as
unsigned, then negative offsets would be completely broken.

The DWARF specification version 3 mentions in the Data Representation
section, page 127:

> If one of the DW_FORM_data<n> forms is used to represent a signed or
unsigned integer, it can be hard for a consumer to discover the context
necessary t...