Now that we check visibility attributes in an appropriate order,

there is no need for mergeVisibily to ever increase the visibility. Not
doing so lets us replace an incorrect use of mergeVisibilityWithMin. The
testcase

struct HIDDEN RECT {
  int top;
};
DEFAULT RECT foo = {0};

shows that we should give preference to one of the attributes instead of
keeping the minimum. We still get this testcase wrong because mergeVisibily
handles two explicit visibilities incorrectly, but this is a step in the
right direction.

llvm-svn: 155101