Allow constant expressions in pragma loop hints.

Previously loop hints such as #pragma loop vectorize_width(#) required a constant. This patch allows a constant expression to be used as well. Such as a non-type template parameter or an expression (2 * c + 1).

Reviewed by Richard Smith


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219589 91177308-0d34-0410-b5e6-96231b3b80d8