Remove overzealous verifier check on DW_OP_LLVM_entry_value and improve the documentation

Based on the comments in the code, the idea is that AsmPrinter is
unable to produce entry value blocks of arbitrary length, such as
DW_OP_entry_value [DW_OP_reg5 DW_OP_lit1 DW_OP_plus]. But the way the
Verifier check is written it also disallows DW_OP_entry_value
[DW_OP_reg5] DW_OP_lit1 DW_OP_plus which seems to overshoot the
target.

Note that this patch does not change any of the safety guards in
LiveDebugValues — there is zero behavior change for clang. It just
allows us to legalize more complex expressions in future patches.

rdar://73907559

Differential Revision: https://reviews.llvm.org/D95990