[flang][OpenMP] Rename "declare constructs" to directives, NFC (#194240)
Only executable directives are constructs in OpenMP, so, for example,
"declare mapper" is not a construct.
Apply
find flang/ \( -name '*.cpp' -o -name '*.h' -o -name '*.f90' \) -exec sed \
-i -E -e 's/OpenMP(Declare[A-Za-z]*)Construct\b/Omp\1Directive/g' {} \;
plus local formatting updates as needed.