[modules] When using -E, we may try to merge decls despite having no Sema

object. In such a case, use the TU's DC for merging global decls rather than
giving up when we find there is no TU scope.

Ultimately, we should probably avoid all loading of decls when preprocessing,
but there are other reasonable use cases for loading an AST file with no Sema
object for which this is the right thing.

llvm-svn: 228234