Have getOrCreateSubprogramDIE store the DIE for a subprogram

Have getOrCreateSubprogramDIE store the DIE for a subprogram
definition in the map before calling itself to retrieve the
DIE for the declaration.  Without this change, if this causes
getOrCreateSubprogramDIE to be recursively called on the definition,
it will create multiple DIEs for that definition.  Fixes PR12831.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157541 91177308-0d34-0410-b5e6-96231b3b80d8