Revert "[flang] Add & use a better visit()"

This reverts commit 2ab9990c9eb79682a4d4b183dfbc7612d3e55328. It has
caused multiple build failures:
*  https://lab.llvm.org/buildbot/#/builders/177/builds/4346
*  https://lab.llvm.org/buildbot/#/builders/180/builds/3803
*  https://lab.llvm.org/buildbot/#/builders/175/builds/10419
*  https://lab.llvm.org/buildbot/#/builders/191/builds/4318
*  https://lab.llvm.org/buildbot/#/builders/173/builds/4274
*  https://lab.llvm.org/buildbot/#/builders/181/builds/4297

All these bots failed with a time-out:
```
command timed out: 1200 seconds without output running [b'ninja', b'-j', b'32'], attempting to kill
```
I'm guessing that that's due to template instantiations failing at some
point (https://reviews.llvm.org/D122441 introduced a custom
implementation of std::visit). Everything seems fine when either:
* building on X86 with GCC or Clang (tested with GCC 9.3 and Clang 12)
* building on AArch64 with GCC (tested with GCC 11)

1 files changed, 15 insertions, 15 deletions

diff --git a/flang/lib/Semantics/program-tree.cpp b/flang/lib/Semantics/program-tree.cpp
index e96a6c4..9d76cfa 100644
--- a/flang/lib/Semantics/program-tree.cpp
+++ b/flang/lib/Semantics/program-tree.cpp
@@ -88,7 +88,7 @@ static ProgramTree BuildSubprogramTree(const parser::Name &name, const T &x) {
   if (subps) {
     for (const auto &subp :
         std::get<std::list<parser::InternalSubprogram>>(subps->t)) {
-      common::visit(
+      std::visit(
           [&](const auto &y) { node.AddChild(ProgramTree::Build(y.value())); },
           subp.u);
     }
@@ -111,7 +111,7 @@ static ProgramTree BuildModuleTree(const parser::Name &name, const T &x) {
   if (subps) {
     for (const auto &subp :
         std::get<std::list<parser::ModuleSubprogram>>(subps->t)) {
-      common::visit(
+      std::visit(
           [&](const auto &y) { node.AddChild(ProgramTree::Build(y.value())); },
           subp.u);
     }
@@ -120,7 +120,7 @@ static ProgramTree BuildModuleTree(const parser::Name &name, const T &x) {
 }
 
 ProgramTree ProgramTree::Build(const parser::ProgramUnit &x) {
-  return common::visit([](const auto &y) { return Build(y.value()); }, x.u);
+  return std::visit([](const auto &y) { return Build(y.value()); }, x.u);
 }
 
 ProgramTree ProgramTree::Build(const parser::MainProgram &x) {
@@ -200,17 +200,17 @@ Symbol::Flag ProgramTree::GetSubpFlag() const {
 
 bool ProgramTree::HasModulePrefix() const {
   using ListType = std::list<parser::PrefixSpec>;
-  const auto *prefixes{common::visit(
-      common::visitors{
-          [](const parser::Statement<parser::FunctionStmt> *x) {
-            return &std::get<ListType>(x->statement.t);
-          },
-          [](const parser::Statement<parser::SubroutineStmt> *x) {
-            return &std::get<ListType>(x->statement.t);
-          },
-          [](const auto *) -> const ListType * { return nullptr; },
-      },
-      stmt_)};
+  const auto *prefixes{
+      std::visit(common::visitors{
+                     [](const parser::Statement<parser::FunctionStmt> *x) {
+                       return &std::get<ListType>(x->statement.t);
+                     },
+                     [](const parser::Statement<parser::SubroutineStmt> *x) {
+                       return &std::get<ListType>(x->statement.t);
+                     },
+                     [](const auto *) -> const ListType * { return nullptr; },
+                 },
+          stmt_)};
   if (prefixes) {
     for (const auto &prefix : *prefixes) {
       if (std::holds_alternative<parser::PrefixSpec::Module>(prefix.u)) {
@@ -222,7 +222,7 @@ bool ProgramTree::HasModulePrefix() const {
 }
 
 ProgramTree::Kind ProgramTree::GetKind() const {
-  return common::visit(
+  return std::visit(
       common::visitors{
           [](const parser::Statement<parser::ProgramStmt> *) {
             return Kind::Program;