Change constantexpr global variable initializers to convert the constants

Change constantexpr global variable initializers to convert the constants
to MCExpr then emit them through MCStreamer with EmitValue.  I think all
global variable initializers are now going through mcstreamer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94293 91177308-0d34-0410-b5e6-96231b3b80d8