MachineModuleInfo: Initialize DbgInfoAvailable depending on debug_cus existing

Before this patch DbgInfoAvailable was set to true in
DwarfDebug::beginModule() or CodeViewDebug::CodeViewDebug(). This made
MIR testing weird since passes would suddenly stop dealing with debug
info just because we stopped the pipeline before the debug printers.

This patch changes the logic to initialize DbgInfoAvailable based on the
fact that debug_compile_units exist in the llvm Module. The debug
printers may then override it with false in case of debug printing being
disabled.

Differential Revision: https://reviews.llvm.org/D53885

llvm-svn: 345740