[lldb] Fix crash when tab-completing in multi-line expr

Summary:
Tab completing inside the multiline expression command can cause LLDB to crash. The easiest way
to do this is to go inside a frame with at least one local variable and then try to complete:

    (lldb) expr
    1. a[tab]

Reason for this was some mixup when we calculate the cursor position. Obviously we should calculate
the offset inside the string by doing 'end - start', but we are doing 'start - end' (which causes the offset to
become -1 which will lead to some out-of-bounds reading).

Fixes rdar://51754005

I don't see any way to test this as the *multiline* expression completion is completely untested at the moment
and I don't think we have any existing code for testing infrastructure for it.

Reviewers: shafik, davide, labath

Reviewed By: labath

Subscribers: abidh, lldb-commits, davide, clayborg, labath

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64995

llvm-svn: 367308