[flang][NFC] Extract name resolution for OpenACC & OpenMP into new file

Move `ResolveAccParts` and `ResolveOmpParts` from resolve-names.cpp to
resolve-directives.{h,cpp}. Move the implementation in the classes
`DirectiveAttributeVisitor`, `AccAttributeVisitor`, and
`OmpAttributeVisitor` to resolve-directives.cpp as well.

To allow this to happen, move `EvaluateIntExpr` and introduce
`EvaluateInt64` to resolve-names-utils.h. The latter is also useful
elsewhere in resolve-names.cpp for converting an Expr to std::int64_t.

The other problem was that `ResolveDesignator` was called from the code
that was moved. At the moment it doesn't seem to be doing anything so I
removed the calls (and no tests failed). If it proves to be needed, we
can either resolve those designators in resolve-names.cpp or pass the
`ResolveDesignator` function in to the code that needs to call it.

Differential Revision: https://reviews.llvm.org/D84768