Debug Info: Fix a problem that resulted in missing DW_AT_specifications
for C++ constructors.

If the DIType for a class was generated by
CGDebugInfo::createContextChain(), the cache contains only a
limited DIType wihtout any declarations. Since EmitFunctionStart()
needs to find the canonical declaration for each method, we
construct the complete type before emitting any method.

rdar://problem/13116508