Emit standard-library RTTI with external linkage, not weak_odr.

Emit standard-library RTTI with external linkage, not weak_odr.

Apply hidden visibility to most RTTI;  libstdc++ does not rely on exact
pointer equality for the type info (just the type info names).  Apply
the same optimization to RTTI that we do to vtables.

Fixes PR5962.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110192 91177308-0d34-0410-b5e6-96231b3b80d8