[flang][msvc] Explicitly reference "this" inside closure. NFC.

[Meinersbur]

Summary

The Microsoft compiler seems to have difficulties to decide between a const/non-const method of a captured object context in a closure. The error message is:

symbol.cpp(261): error C2668: 'Fortran::semantics::Symbol::detailsIf': ambiguous call to overloaded function
symbol.h(535): note: could be 'const D *Fortran::semantics::Symbol::detailsIf<Fortran::semantics::DerivedTypeDetails>(void) const'
symbol.h(534): note: or       'D *Fortran::semantics::Symbol::detailsIf<Fortran::semantics::DerivedTypeDetails>(void)'
symbol.cpp(261): note: while trying to match the argument list '()'

Explicitly using the this-pointer resolves this problem.

This patch is part of the series to make flang compilable with MS Visual Studio http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html.

[Meinersbur]

Visual Studio bug report: https://developercommunity.visualstudio.com/content/problem/1192775/ambiguous-constnon-const-method-call-in-implicit-t.html

[DavidTruby]

LGTM

flang/lib/Semantics/symbol.cpp

return std::visit(
         common::visitors{
             [](const UseErrorDetails &) { return true; },
             [&](const ObjectEntityDetails &) { return has<EntityDetails>(); },
             [&](const ProcEntityDetails &) { return has<EntityDetails>(); },
             [&](const SubprogramDetails &) {
               return has<SubprogramNameDetails>() || has<EntityDetails>();
             },
             [&](const DerivedTypeDetails &) {
-              auto *derived{detailsIf<DerivedTypeDetails>()};
+              auto *derived{this->detailsIf<DerivedTypeDetails>()};
               return derived && derived->isForwardReferenced();
             },
             [](const auto &) { return false; },
         },
         details);
   }
 }