fix bug in reattaching debugger for asm.js

[MikeHolman]

Resolves #1
Debugger attach->detach->attach scenario is broken for asm.js. Some fields need to be cleaned up but aren't (notably AsmJsFunctionInfo). This scenario is most commonly hit through using the performance tab of f12 on pages with asm.js.

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[akroshg]

GetFunctionBody() guarantees to return non-null?

[MikeHolman]

To be in this list, scriptFunction->GetFunctionBody()->GetAsmJsFunctionInfo() must be non-null (from TransitionEnvironmentForDebugger). But that also assumes GetFunctionBody() is non-null.

From comment "// We should have force parsed the function, and have a function body" in RecyclerFunctionCallbackForDebugger, and my understanding that we full parse everything when we attach debugger, I took this as truth. If you want, I can add an assert here/in TransitionEnvironmentForDebugger, if nothing else but for the sake of clarity.

[akroshg]

LGTM