Do not consider Shared symbols as defined symbols.

I don't remember why I made shared symbols one type of defined symbols.
Shared symbols aren't undefined, so it could be considered defined, but
categorizing three symbols as:

 - defined
   - really defined
   - shared
 - undefined

is not as intuitive as

 - defined
 - shared
 - undefined

to me. So, in this patch, I made a change to stop handling shared
symbols as defined symbols.

Surprisingly, I didn't have to update any tests for this change.

Differential Revision: https://reviews.llvm.org/D39394

llvm-svn: 317446