[flang][OpenMP] Lowering support for default clause

This patch adds lowering support for default clause.

1. During symbol resolution in semantics, should the enclosing context have
a default data sharing clause defined and a `parser::Name` is not attached
to an explicit data sharing clause, the
`semantics::Symbol::Flag::OmpPrivate` flag (in case of default(private))
and `semantics::Symbol::Flag::OmpFirstprivate` flag (in case of
default(firstprivate)) is added to the symbol.

2. During lowering, all symbols having either
`semantics::Symbol::Flag::OmpPrivate` or
`semantics::Symbol::Flag::OmpFirstprivate` flag are collected and
privatised appropriately.

Co-authored-by: Peixin Qiao <qiaopeixin@huawei.com>

Reviewed by: peixin

Differential Revision: https://reviews.llvm.org/D123930