[msan] Don't unpoison phdrs on dlopen(NULL, 0)

Summary:
dlopen(NULL, ...) is intended to give you back a handle to the
executable for use with dlsym. Casting it to link_map and using it with
ForEachMappedRegion results in a crash.

We also shouldn't unpoison the globals of a DSO that is already in
memory. This ensures that we don't do it for the executable, but in
general, MSan may have false negatives if the DSO is already loaded.

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14795

llvm-svn: 253530