[flang] [OpenMP] Implement no-multiple-appearance rule for DSA Clauses

[flang] [OpenMP] Implement no-multiple-appearance rule for DSA Clauses

DSA stands for Data-Sharing Attribute. This work is part of the
Name Resolution for OpenMP framework (data-refs on clauses part)

Based on 2.15.3: A list item that specifies a given variable may
not appear in more than one clause on the same directive, except that
a variable may be specified in both firstprivate and lastprivate clauses.

Through a temporary `std::set` of `const Symbol *` to keep track of
all the symbols on a certain OpenMP directive, we can determine whether
a variable `Name` (or `Symbol`, more accurately) has already appeared on
another DSA clause already, with the exception of FIRSTPRIVATE clause
and LASTPRIVATE clause. This rule applies to `/COMMON block/` also and
the source provenance shows on error message points to the `Name` between
slashes.

Added two more tests and changed some existing tests to accommodate
this commit. I intend to keep the `omp-clause-validity01.f90` test to
do the validity checks only.

Original-commit: flang-compiler/f18@0d7828c21e8dfe02e470a861178b7747100516ae