[flang][NFC] Remove obsolete DoLoopHelper

During the upstreaming process from fir-dev some
new builder have been introduced in the `flang/Optimizer/Builder`
directory. This patch removes the obsolete DoLoopHelper still present
in the lowering directories and makes use of the new one where needed.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D118442

flang/lib/Lower/CMakeLists.txt

@@ -6,7 +6,6 @@ add_flang_library(FortranLower
   Coarray.cpp
   ComplexExpr.cpp
   ConvertType.cpp
-  DoLoopHelper.cpp
   IntrinsicCall.cpp
   IO.cpp
   Mangler.cpp

flang/lib/Lower/CharacterExpr.cpp

@@ -8,8 +8,8 @@
 
 #include "flang/Lower/CharacterExpr.h"
 #include "flang/Lower/ConvertType.h"
-#include "flang/Lower/DoLoopHelper.h"
 #include "flang/Lower/IntrinsicCall.h"
+#include "flang/Optimizer/Builder/DoLoopHelper.h"
 
 //===----------------------------------------------------------------------===//
 // CharacterExprHelper implementation
@@ -179,7 +179,7 @@ void Fortran::lower::CharacterExprHelper::createStoreCharAt(
 void Fortran::lower::CharacterExprHelper::createCopy(
     const fir::CharBoxValue &dest, const fir::CharBoxValue &src,
     mlir::Value count) {
-  Fortran::lower::DoLoopHelper{builder, loc}.createLoop(
+  fir::factory::DoLoopHelper{builder, loc}.createLoop(
       count, [&](fir::FirOpBuilder &, mlir::Value index) {
         auto charVal = createLoadCharAt(src, index);
         createStoreCharAt(dest, index, charVal);
@@ -191,7 +191,7 @@ void Fortran::lower::CharacterExprHelper::createPadding(
   auto blank = createBlankConstant(getCharacterType(str));
   // Always create the loop, if upper < lower, no iteration will be
   // executed.
-  Fortran::lower::DoLoopHelper{builder, loc}.createLoop(
+  fir::factory::DoLoopHelper{builder, loc}.createLoop(
       lower, upper, [&](fir::FirOpBuilder &, mlir::Value index) {
         createStoreCharAt(str, index, blank);
       });
@@ -286,7 +286,7 @@ fir::CharBoxValue Fortran::lower::CharacterExprHelper::createConcatenate(
   auto upperBound = builder.create<mlir::arith::SubIOp>(loc, len, one);
   auto lhsLen =
       builder.createConvert(loc, builder.getIndexType(), lhs.getLen());
-  Fortran::lower::DoLoopHelper{builder, loc}.createLoop(
+  fir::factory::DoLoopHelper{builder, loc}.createLoop(
       lhs.getLen(), upperBound, one,
       [&](fir::FirOpBuilder &bldr, mlir::Value index) {
         auto rhsIndex = bldr.create<mlir::arith::SubIOp>(loc, index, lhsLen);