[OPENMP] Fix for checking of data-sharing attributes for canonical var decls only.

[OPENMP] Fix for checking of data-sharing attributes for canonical var decls only.

Currently checks for active data-sharing attributes for variables are performed for found var decls. Instead these checks must be performed for canonical decls of these variables to avoid possible troubles with with the differently qualified re-declarations of the same variable, for example:
namespace A { int x; }
namespace B { using A::x; }
Both A::x and B::x actually reference the same object A::x and this fact must be taken into account during data-sharing attributes analysis.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@235096 91177308-0d34-0410-b5e6-96231b3b80d8