[BOLT][DWARF] Fix handling of invalid DIE references

Compiler can generate DIE References that are invalid. Previously BOLT could
assert when writing out IR to .debug_info. Changed where DIE offsets are changed
so that it's always done. Thus making sure that assert is not triggered.

Added more specific warnings, and ability to print out invalid referenced DIE
offset when verbosity >=1.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D157746