[lldb] Remove redundant severity substring within a diagnostic message. (#76111)

For example, the following message has the severity string "error: "
twice.
	> "error: <EXPR>:3:1: error: cannot find 'bogus' in scope

This method already appends the severity string in the beginning, but
with this fix, it also removes a secondary instance, if applicable.

Note that this change only removes the *first* redundant substring. I
considered putting the removal logic in a loop, but I decided that if
something is generating more than one redundant severity substring, then
that's a problem the message's source should probably fix.

rdar://114203423