[LLDB/Lua] add support for one-liner breakpoint callback
These callbacks are set using the following:
breakpoint command add -s lua -o "print('hello world!')"
The user supplied script is executed as:
function (frame, bp_loc, ...)
<body>
end
So the local variables 'frame', 'bp_loc' and vararg are all accessible.
Any global variables declared will persist in the Lua interpreter.
A user should never hold 'frame' and 'bp_loc' in a global variable as
these userdatas are context dependent.
Differential Revision: https://reviews.llvm.org/D91508