Fix undefined behavior in Variable.h

m_loc_is_constant_data was uninitialized, so unless someone
explicitly called SetLocIsConstantData(), this would be UB.

I think every existing call-site would always call the proper
function to initialize the value, so there were no existing
bugs, but I encountered this when I tried to use it without
calling this function and encountered this.

llvm-svn: 348813