[flang][OpenMP] MSVC buildbot fix

PR llvm#153488 caused the msvc build (https://lab.llvm.org/buildbot/#/builders/166/builds/1397) to fail:
```
..\llvm-project\flang\include\flang/Evaluate/rewrite.h(78): error C2668: 'Fortran::evaluate::rewrite::Identity::operator ()': ambiguous call to overloaded function
..\llvm-project\flang\include\flang/Evaluate/rewrite.h(43): note: could be 'Fortran::evaluate::Expr<Fortran::evaluate::SomeType> Fortran::evaluate::rewrite::Identity::operator ()<Fortran::evaluate::SomeType,S>(Fortran::evaluate::Expr<Fortran::evaluate::SomeType> &&,const U &)'
        with
        [
            S=Fortran::evaluate::value::Integer<128,true,32,unsigned int,unsigned __int64,128>,
            U=Fortran::evaluate::value::Integer<128,true,32,unsigned int,unsigned __int64,128>
        ]
..\llvm-project\flang\lib\Semantics\check-omp-atomic.cpp(174): note: or       'Fortran::evaluate::Expr<Fortran::evaluate::SomeType> Fortran::semantics::ReassocRewriter::operator ()<Fortran::evaluate::SomeType,S,void>(Fortran::evaluate::Expr<Fortran::evaluate::SomeType> &&,const U &,Fortran::semantics::ReassocRewriter::NonIntegralTag)'
        with
        [
            S=Fortran::evaluate::value::Integer<128,true,32,unsigned int,unsigned __int64,128>,
            U=Fortran::evaluate::value::Integer<128,true,32,unsigned int,unsigned __int64,128>
        ]
..\llvm-project\flang\include\flang/Evaluate/rewrite.h(78): note: while trying to match the argument list '(Fortran::evaluate::Expr<Fortran::evaluate::SomeType>, const S)'
        with
        [
            S=Fortran::evaluate::value::Integer<128,true,32,unsigned int,unsigned __int64,128>
        ]
..\llvm-project\flang\include\flang/Evaluate/rewrite.h(78): note: the template instantiation context (the oldest one first) is
..\llvm-project\flang\lib\Semantics\check-omp-atomic.cpp(814): note: see reference to function template instantiation 'U Fortran::evaluate::rewrite::Mutator<Fortran::semantics::ReassocRewriter>::operator ()<const Fortran::evaluate::Expr<Fortran::evaluate::SomeType>&,Fortran::evaluate::Expr<Fortran::evaluate::SomeType>>(T)' being compiled
        with
        [
            U=Fortran::evaluate::Expr<Fortran::evaluate::SomeType>,
            T=const Fortran::evaluate::Expr<Fortran::evaluate::SomeType> &
        ]
```

The reason is that there is an ambiguite between operator() of
ReassocRewriter itself as operator() of the base class Identity through
`using Id::operator();`. By the C++ specification, method declarations
in ReassocRewriter hide methods with the same signature from a using
declaration, but this does not apply to
```
evaluate::Expr<T> operator()(..., NonIntegralTag = {})
```
which has a different signature due to an additional tag parameter.
Since it has a default value, it is ambiguous with operator() without
tag parameter.

GCC and Clang both accept this, but in my understanding MSVC is correct
here.

Since the overloads of ReassocRewriter cover all cases, remopving the
using reclarations to avoid the ambiguity.

Author: Meinersbur

flang/lib/Semantics/check-omp-atomic.cpp

@@ -86,7 +86,6 @@ ReassocOp<T, Op0, Op1> reassocOp(const Op0 &op0, const Op1 &op1) {
 
 struct ReassocRewriter : public evaluate::rewrite::Identity {
   using Id = evaluate::rewrite::Identity;
-  using Id::operator();
   struct NonIntegralTag {};
 
   ReassocRewriter(const SomeExpr &atom) : atom_(atom) {}