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: http://llvm.org/svn/llvm-project/cfe/trunk@205668 91177308-0d34-0410-b5e6-96231b3b80d8
Reapply r205655, DebugInfo: Place global constants in their appropriate context.

This was reverted in 205664 and seems to be fixed by 205668... though
that may be more by accident than anything well founded.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@205669 91177308-0d34-0410-b5e6-96231b3b80d8