[llgo] debug: create replaceable type through DIBuilder

Summary:
llgo was asserting in DebugInfo, which was interpreting
the temporary MDNodes we were creating as DIScopes instead
of DITypes (in DIScope::getRef).

This proposal changes llgo to use DIBuilder's
createReplaceableCompositeType method to create a DIType
that can be RAUW'd.

Reviewers: pcc

Reviewed By: pcc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7852

llvm-svn: 230953