Implemented xdebug_get_function_stack

Summary: This function adds the xdebug function xdebug_get_function_stack.
xdebug_get_function_stack returns an array representing the stack
at the time xdebug_get_function_stack was called.

I split the function into a php portion and a native portion. The php
portion just manipulates the output of the native portion in order to
match the output of php5 xdebug. Since there was a lot of array manipulation
involved, I felt that php code would be more clear.

The native portion just calls debugBacktrace, but I had to modify
debugBacktrace to optionally include pseudoMain and function argument names.

Reviewed By: @ptarjan

Differential Revision: D1407595