Allow the type of a global to be different from the type of its

associated declaration. This is a prerequisite to handling
general union initializations; for example, an array of unions involving 
pointers has to be turned into a struct because the elements can have 
incompatible types.

I refactored the code a bit to make it more readable; now, the logic for 
definitions is all in EmitGlobalVarInit.

The second parameter for GetAddrOfGlobalVar is now dead; I'll remove it 
separately.

By itself, this patch should not cause any visible changes.

llvm-svn: 51783