[OpenMP] Fix infinite loop in Sema::isOpenMPGlobalCapturedDecl()

Function Sema::isOpenMPGlobalCapturedDecl() has a parameter `unsigned Level`,
but use `Level >= 0` as the condition of `while`, thus cause an infinite loop.
Fix by changing the loop condition to `Level > 0`.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D86858