MachineModuleInfo: Don't allow dynamically setting DbgInfoAvailable

This can be set up front, and used only as a cache. This avoids a
field that looks like it requires MIR serialization.

I believe this fixes 2 bugs for CodeView. First, this addresses a
FIXME that the flag -diable-debug-info-print only works with
DWARF. Second, it fixes emitting debug info with emissionKind NoDebug.