[flang] Add & use a better visit() (take 2)

Adds flang/include/flang/Common/log2-visit.h, which defines
a Fortran::common::visit() template function that is a drop-in
replacement for std::visit().  Modifies most use sites in
the front-end and runtime to use common::visit().

The C++ standard mandates that std::visit() have O(1) execution
time, which forces implementations to build dispatch tables.
This new common::visit() is O(log2 N) in the number of alternatives
in a variant<>, but that N tends to be small and so this change
produces a fairly significant improvement in compiler build
memory requirements, a 5-10% improvement in compiler build time,
and a small improvement in compiler execution time.

Building with -DFLANG_USE_STD_VISIT causes common::visit()
to be an alias for std::visit().

Calls to common::visit() with multiple variant arguments
are referred to std::visit(), pending further work.

This change is enabled only for GCC builds with GCC >= 9;
an earlier attempt (D122441) ran into bugs in some versions of
clang and was reverted rather than simply disabled; and it is
not well tested with MSVC. In non-GCC and older GCC builds,
common::visit() is simply an alias for std::visit().

flang/lib/Semantics/pointer-assignment.cpp

@@ -117,7 +117,7 @@ template <typename T> bool PointerAssignmentChecker::Check(const T &) {
 
 template <typename T>
 bool PointerAssignmentChecker::Check(const evaluate::Expr<T> &x) {
-  return std::visit([&](const auto &x) { return Check(x); }, x.u);
+  return common::visit([&](const auto &x) { return Check(x); }, x.u);
 }
 
 bool PointerAssignmentChecker::Check(const SomeExpr &rhs) {
@@ -128,7 +128,7 @@ bool PointerAssignmentChecker::Check(const SomeExpr &rhs) {
     Say("A coindexed object may not be a pointer target"_err_en_US);
     return false;
   } else {
-    return std::visit([&](const auto &x) { return Check(x); }, rhs.u);
+    return common::visit([&](const auto &x) { return Check(x); }, rhs.u);
   }
 }
 
@@ -322,7 +322,7 @@ static bool CheckPointerBounds(
   const SomeExpr &lhs{assignment.lhs};
   const SomeExpr &rhs{assignment.rhs};
   bool isBoundsRemapping{false};
-  std::size_t numBounds{std::visit(
+  std::size_t numBounds{common::visit(
       common::visitors{
           [&](const evaluate::Assignment::BoundsSpec &bounds) {
             return bounds.size();