[LLDB] Fix script to work with GNU sed

GNU sed and BSD sed have a different command-line syntax for in-place
editing, and the current form of the script would only work with BSD
sed. The easiest way to get cross-platform behavior is to specify a
backup suffix and then just delete the backup file at the end. (BSD sed
is the default on macOS, but it's possible to acquire GNU coreutils and
have your `sed` be GNU sed even on macOS; I'm aware it's not officially
supported in any capacity, but it's easy enough to support here.)

An alternative would be using `perl -p -i -e` instead of `sed -i`, but I
figured it was best to make the minimal working change.

Differential Revision: https://reviews.llvm.org/D51374

llvm-svn: 340885