Editing multi-line content in a terminal environment involves a lot of

trade-offs. When LLDB's multi-line editing support was first introduced
for expressions / REPL contexts the behavior was as follows:

* The Return key is treated as a line-break except at the end of the input
  buffer, where a completeness test is applied

This worked well enough when writing code, and makes it trivial to insert
new lines above code you've already typed. Just use cursor navigation to
move up and type freely. Where it was awkward is that the gesture to insert
a line break and end editing is conflated for most people. Sometimes you
want Return to end the editing session and other times you want to insert
a line break.

This commit changes the behavior as follows:

* The Return key is treated as the end of editing except at the end of the
  input buffer, where a completeness test is applied

* The Meta+Return sequence is always treated as a line break. This is
  consistent with conventions in Facebook and elsewhere since
  Alt/Option+Return is often mapped to Meta+Return. The unfortunate
  exception is on macOS where this *can* be the case, but isn't by
  default. Sigh.

Note that by design both before and after the patch pasting a Return
character always introduces a line break.

<rdar://problem/26886287>

llvm-svn: 275482