[DebugInfo] Do not record artificial global initializer functions in the DeclCache.

When we are generating the global initializer functions, we call
CGDebugInfo::EmitFunctionStart() with a valid decl which is describing
the initialized global variable. Do not update the DeclCache with this
key as it will overwrite the the cached variable DIGlobalVariable with
the newly created artificial DISubprogram.

One could wonder if we should put artificial subprograms in the DIE tree
at all (there are vaild uses for them carrying line information though).