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, 3 insertions, 3 deletions

diff --git a/flang/lib/Semantics/pointer-assignment.cpp b/flang/lib/Semantics/pointer-assignment.cpp
index d55fa16..2b065253 100644
--- a/flang/lib/Semantics/pointer-assignment.cpp
+++ b/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 common::visit([&](const auto &x) { return Check(x); }, x.u);
+  return std::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 common::visit([&](const auto &x) { return Check(x); }, rhs.u);
+    return std::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{common::visit(
+  std::size_t numBounds{std::visit(
       common::visitors{
           [&](const evaluate::Assignment::BoundsSpec &bounds) {
             return bounds.size();