LanguageRuntime: Simplify NSException::GetSummary() output

Summary:
Right now, NSException::GetSummary() has the following output:
"name: $exception_name - reason: $exception_reason"

It would be better to simplify the output by removing the name and only
showing the exception's reason. This way, annotations would look nicer in
the editor, and would be a shorter summary in the Variables Inspector.

Accessing the exception's name can still be done by expanding the
NSException object in the Variables Inspector.

rdar://54770115



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D71311



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>