[MachineInstrBuilder] Introduce MIMetadata to simplify metadata propagation

In many places DebugLoc and PCSections metadata are just copied along to
propagate them through MachineInstrs. Simplify doing so by bundling them
up in a MIMetadata class that replaces the DebugLoc argument to most
BuildMI() variants.

The DebugLoc-only constructors allow implicit construction, so that
existing usage of `BuildMI(.., DL, ..)` works as before, and the rest of
the codebase using BuildMI() does not require changes.

NFC.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D130883