[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().

1 files changed, 1 insertions, 1 deletions

diff --git a/flang/lib/Semantics/check-nullify.cpp b/flang/lib/Semantics/check-nullify.cpp
index 9ee45541fd5c..6b3c8d4daf0f 100644
--- a/flang/lib/Semantics/check-nullify.cpp
+++ b/flang/lib/Semantics/check-nullify.cpp
@@ -23,7 +23,7 @@ void NullifyChecker::Leave(const parser::NullifyStmt &nullifyStmt) {
   parser::ContextualMessages messages{
       *context_.location(), &context_.messages()};
   for (const parser::PointerObject &pointerObject : nullifyStmt.v) {
-    std::visit(
+    common::visit(
         common::visitors{
             [&](const parser::Name &name) {
               const Symbol *symbol{name.symbol};