dsymutil: support dwarf version mismatches between object and clang module

This adds a missing call to maybeUpdateMaxDwarfVersion when visitng a
clang module. Failing to do so will cause a failure when emitting
DWARF 4 forms into a CU that AsmPrinter believes to be DWARF 2.

rdar://problem/33666528

llvm-svn: 310392