Read symbols from the debugger backend

[xusheng6]

There are certain symbols that the debugger (windbg/lldb) understands but BN does not know about. For example, when a register points to the address of a Windows API function, it would be great if we can let the user know about it. However, this requires the debugger knowing the address of the API function, which is not present in the executable binary view. To solve this problem, we need to read symbols from the debugger backend and somehow integrate it into the debugger binary view. So the annotation process is aware of it.

[xusheng6]

So a compromise on this is that we can offer a method to read the symbol at a specific address. The users can use this API on demand. This avoids many tricky design issues, e.g., how often/when should we update the symbols.