[lldb] Fix step-avoid-regexp logging

Ensure step-avoid-regexp logs are emitted in the case where the regex has no
capture groups.

Without this change, the log is printed only if the regex has at least one
capture group.

Another change is to the log message: the first capture group has been removed
from the message. There could be zero capture groups, and there could be two or
more capture groups.

Differential Revision: https://reviews.llvm.org/D119298