dsymutil: strip unused types from imported DW_TAG_modules

This patch teaches dsymutil to strip types from the imported
DW_TAG_module inside of an object file (not inside the PCM) if they
can be resolved to the full definition inside the PCM. This reduces
the size of the .dSYM from WebCore from webkit.org by almost 2/3.

<rdar://problem/33047213>

llvm-svn: 308710