swift234

[Global Opt] replace GlobalAddrInst with SILGlobalAddrInst.

Update SILGen to create SILGlobalVariable and SILGlobalAddrInst instead of
GlobalAddrInst. When we see a definition for a global variable, we create
the corrsponding SILGlobalVariable definition.

When creating SILGlobalVariable from a global VarDecl, we mangle the global
VarDecl in the same way as we mangle it at IRGen. The SILLinkage is also
set in the same way as we set it at IRGen.

At IRGen, we use the associated VarDecl for SILGlobalVariable if it exists,
to have better debugging information.

We set the initializer for SILGlobalVariable definition only.

We also handle SILGlobalAddrInst in various SILPasses, in the similar way
as we handle GlobalAddrInst.

rdar://15493694


Swift SVN r21887