crashlog.py: Improve regular expressions

This is yet another change to the regular expressions in crashlog.py
that fix a few edge cases, and attempt to improve the readability
quite a bit in the process. My last change to support spaces in
filenames introduced a bug that caused the version/archspec field to
be parsed as part of the image name.

For example, in "0x1111111 - 0x22222 +MyApp Pro arm64 <01234>", the
name of the image was recognized as "MyApp Pro arm64" instead of
"MyApp Pro" with a "version" of arm64.

The bugfix makes the space following an optional field mandatory
*inside* the optional group.

rdar://problem/56883435

Differential Revision: https://reviews.llvm.org/D69871