Move the point in the code where we handle -fvisibility=hidden. With

Move the point in the code where we handle -fvisibility=hidden. With
the current implementation this should be a nop as explicit visibility
takes precedence in mergeVisibility.

The location chosen is such that attributes checked above it can force
a symbol to be default. For example, an attribute is the variable or function.
Attributes checked after this point, can only make the visibility more
restrictive. An attribute in a type for example.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@155098 91177308-0d34-0410-b5e6-96231b3b80d8