[AMDGPU][NFC] DWARF vector composite location description operations (#71623)

Summary:
Add description to AMDGPUDwarfExtensionsForHeterogeneousDebugging.rst
for "DWARF Operations to Create Vector Composite Location Descriptions"
proposal to explain the main motivation is to facilitate more compact
DWARF that is faster to evaluate.

Reviewers: kzhuravl, scott.linder, zoran.zaric

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