[AArch64][SVE] Emit DWARF location expression for SVE stack objects.

Extend PEI to emit a DWARF expression for StackOffsets that have
a fixed and scalable component. This means the expression that needs
to be added is either:
  <base> + offset
or:
  <base> + offset + scalable_offset * scalereg

where for SVE, the scale reg is the Vector Granule Dwarf register, which
encodes the number of 64bit 'granules' in an SVE vector and which
the debugger can evaluate at runtime.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D90020