Reduce header footprint of Target.h

This continues the effort to reduce header footprint and improve
build speed by removing clang and other unnecessary headers
from Target.h.  In one case, some headers were included solely
for the purpose of declaring a nested class in Target, which was
not needed by anybody outside the class.  In this case the
definition and implementation of the nested class were isolated
in the .cpp file so the header could be removed.

llvm-svn: 231107