[lldb-mi] Implement -gdb-set breakpoint pending on/off

Summary:
This allows creating pending breakpoint automatically when a location is not found.
This is used by some front-ends instead of doing "-break-insert -f" every time.

See also https://sourceware.org/gdb/onlinedocs/gdb/Set-Breaks.html



Signed-off-by: Marc-Andre Laperle <malaperle@gmail.com>

Subscribers: MaskRay, llvm-commits, lldb-commits, ki.stfu

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D52953

llvm-svn: 345563