[1.9>master] [1.8>1.9] [MERGE #4681 @leirocks] more xdata fix for debugger attaching/detaching
[1.9>master] [1.8>1.9] [MERGE #4681 @leirocks] more xdata fix for debugger attaching/detaching

Merge pull request #4681 from leirocks:xdatadebugger

codegen can happen before OnDebuggerAttach/Detach returns, we need to
clear the function table before the code gen happens.

Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore