Reviewed by Geoffrey Garen.

        Support throwing away non-running code even while other code is running
        https://bugs.webkit.org/show_bug.cgi?id=63485

        Add a function to CodeBlock to support unlinking direct linked callsites,
        and then with that in place add logic to discard code from any function
        that is not currently on the stack.

        The unlinking completely reverts any optimized call sites, such that they
        may be relinked again in future.

        * JavaScriptCore.exp:
        * bytecode/CodeBlock.cpp:
        (JSC::CodeBlock::unlinkCalls):
        (JSC::CodeBlock::clearEvalCache):
        * bytecode/CodeBlock.h:
        (JSC::CallLinkInfo::CallLinkInfo):
        (JSC::CallLinkInfo::unlink):
        * bytecode/EvalCodeCache.h:
        (JSC::EvalCodeCache::clear):
        * heap/Heap.cpp:
        (JSC::Heap::getConservativeRegisterRoots):
        * heap/Heap.h:
        * jit/JIT.cpp:
        (JSC::JIT::privateCompile):
        * jit/JIT.h:
        * jit/JITCall.cpp:
        (JSC::JIT::compileOpCall):
        * jit/JITWriteBarrier.h:
        (JSC::JITWriteBarrierBase::clear):
        * jsc.cpp:
        (GlobalObject::GlobalObject):
        (functionReleaseExecutableMemory):
        * runtime/Executable.cpp:
        (JSC::EvalExecutable::unlinkCalls):
        (JSC::ProgramExecutable::unlinkCalls):
        (JSC::FunctionExecutable::discardCode):
        (JSC::FunctionExecutable::unlinkCalls):
        * runtime/Executable.h:
        * runtime/JSGlobalData.cpp:
        (JSC::SafeRecompiler::returnValue):
        (JSC::SafeRecompiler::operator()):
        (JSC::JSGlobalData::releaseExecutableMemory):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@89885 268f45cc-cd09-0410-ab3c-d52691b4dbfc