[DwarfExpression] Support entry values for indirect parameters

Summary:
A struct argument can be passed-by-value to a callee via a pointer to a
temporary stack copy. Add support for emitting an entry value DBG_VALUE
when an indirect parameter DBG_VALUE becomes unavailable. This is done
by omitting DW_OP_stack_value from the entry value expression, to make
the expression describe the location of an object.

rdar://63373691

Reviewers: djtodoro, aprantl, dstenb

Subscribers: hiraditya, lldb-commits, llvm-commits

Tags: #lldb, #llvm

Differential Revision: https://reviews.llvm.org/D80345