Cleanup code and fix crash on shutdown/exit for clang builds.

We move CppUtils::ReplaceCurrentAccess's logic to a file-local
MainThreadHelper in context.cpp. This fixes the crash for me:

...
********* Finished testing of TestCppAssistants *********
./cpp/tests/cppassistantstest.shell: line 4: 22878 Segmentation fault

GDB showed that this is in the global static dtor of
CppUtils::ReplaceCurrentAccess.