Better solution: calculate the visibility of functions and variables

Better solution:  calculate the visibility of functions and variables
independently of whether they're definitions, then teach IR generation to
ignore non-explicit visibility when emitting declarations.  Use this to
make sure that RTTI, vtables, and VTTs get the right visibility.

More of rdar://problem/8613093

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@117781 91177308-0d34-0410-b5e6-96231b3b80d8