[DebugInfo] Report unsupported maximum_operations_per_instruction values

This patch adds a check which reports an unsupported value of the
maximum_operations_per_instruction field in a debug line table header.
This is reported once per line table, at most, and only if the tablet
would otherwise need to use it (i.e. never for tables with version 3 or
less, or for tables which don't use DW_LNS_const_add_pc or special
opcodes). Unsupported values are currently any apart from 1.

Reviewed by: probinson, MaskRay

Differential Revision: https://reviews.llvm.org/D74819