DebugInfo: Avoid emitting constnants for every use

While the folding set would deduplicate the nodes themselves and LLVM
would handle not emitting the same global twice, it still meant creating
a long/redundant list of global variables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205668 91177308-0d34-0410-b5e6-96231b3b80d8