[MLIR][OpenMP] Add support for declaring critical construct names

Add an operation omp.critical.declare to declare names/symbols of
critical sections. Named omp.critical operations should use symbols
declared by omp.critical.declare. Having a declare operation ensures
that the names of critical sections are global and unique. In the
lowering flow to LLVM IR, the OpenMP IRBuilder creates unique names
for critical sections.

Reviewed By: ftynse, jeanPerier

Differential Revision: https://reviews.llvm.org/D108713