[lldb/Lua] add support for multiline scripted breakpoints

[lldb/Lua] add support for multiline scripted breakpoints

1 - Partial Statements

The interpreter loop runs every line it receives, so partial
Lua statements are not being handled properly. This is a problem for
multiline breakpoint scripts since the interpreter loop, for this
particular case, is just an abstraction to a partially parsed function
body declaration.

This patch addresses this issue and as a side effect improves the
general Lua interpreter loop as well. It's now possible to write partial
statements in the 'script' command.

Example:
   (lldb) script
   >>>   do
   ..>   local a = 123
   ..>   print(a)
   ..>   end
   123

The technique implemented is the same as the one employed by Lua's own REPL implementation.
Partial statements always errors out with the '<eof>' tag in the error
message.

2 - CheckSyntax in Lua.h

In order to support (1), we need an API for just checking the syntax of string buffers.

3 - Multiline scripted breakpoints

Finally, with all the base features implemented this feature is
straightforward. The interpreter loop behaves exactly the same, the
difference is that it will aggregate all Lua statements into the body of
the breakpoint function. An explicit 'quit' statement is needed to exit the
interpreter loop.

Example:
   (lldb) breakpoint command add -s lua
   Enter your Lua command(s). Type 'quit' to end.
   The commands are compiled as the body of the following Lua function
   function (frame, bp_loc, ...) end
   ..> print(456)
   ..> a = 123
   ..> quit

Differential Revision: https://reviews.llvm.org/D93481