LLDB debugger

In open source Swift, the LLDB debugger is both a full-featured debugger for Swift and a read-eval-print-loop (REPL) tool for the language. The LLDB debugger is tightly coupled to the Swift compiler itself, in order for it to inspect Swift types accurately and evaluate expressions correctly. REPL takes advantage of the robust debugging features such as breakpoint settings, interactive context during failures, evaluating expressions, reporting, and formatting results at breakpoints.