Improve llvm-gsymutil performance in quiet mode.

The GSYM code alwasy logging to streams even in quiet mode. When in quiet mode we would use the "nulls()" stream to avoid logging to the terminal, but this still caused logging functions to be called on DWARFDie objects and other messages which were quite expensive and not needed if we weren't logging anything. This patch switches some logs in performant areas to be "raw_ostream *" values and if the ostream pointer is NULL, then we don't call the expensive logging functions on DWARFDie and other objects which will improve performance.

Differential Revision: https://reviews.llvm.org/D157466