Initialize m_allow_cxx, m_allow_objc. These ivars in the base class are not

initialized in the ctor and they're only initialized to 'true' in ClangUserExpression.cpp
when specific languages are detected so we can use uninitialized values.  This
bug has been present since the ivars were added in r144042.
<rdar://problem/31105864> 

llvm-svn: 298333