[flang][OpenMP] Check for OpenMP 5.0+ for the if clause on "do simd"

The "if" clause on "do simd" is only supported from OpenMP version 5.0.
Currently the tablegen for OMP_DoSimd does not restrict it, leading to
an ICE if the clause is used with an older OpenMP version as opposed to
a semantic error. Restrict the version to show the correct semantic error.

Add flang tests for using the if clause with -fopenmp-version=45 to
check for these types of errors in the future.

Signed-off-by: Kajetan Puchalski <[email protected]>

Author: mrkajetanp