Debug Info: Avoid completing class types when a definition is in a module.

This patch adds an early exit to CGDebugInfo::completeClassData() when
compiling with -gmodules and the to-be-completed type is available in
a clang module.

rdar://problem/23599990

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@308938 91177308-0d34-0410-b5e6-96231b3b80d8