[flang][debug] set DW_AT_main_subprogram for fortran main function (#111350)

Requested here
#111022 (comment)

Author: tblah

flang/lib/Optimizer/Transforms/AddDebugInfo.cpp

@@ -267,7 +267,9 @@ void AddDebugInfoPass::handleFuncOp(mlir::func::FuncOp funcOp,
   funcName = mlir::StringAttr::get(context, result.second.name);
 
   // try to use a better function name than _QQmain for the program statement
+  bool isMain = false;
   if (funcName == fir::NameUniquer::doProgramEntry()) {
+    isMain = true;
     mlir::StringAttr bindcName =
         funcOp->getAttrOfType<mlir::StringAttr>(fir::getSymbolAttrName());
     if (bindcName)
@@ -302,6 +304,9 @@ void AddDebugInfoPass::handleFuncOp(mlir::func::FuncOp funcOp,
       mlir::LLVM::DISubprogramFlags{};
   if (isOptimized)
     subprogramFlags = mlir::LLVM::DISubprogramFlags::Optimized;
+  if (isMain)
+    subprogramFlags =
+        subprogramFlags | mlir::LLVM::DISubprogramFlags::MainSubprogram;
   if (!funcOp.isExternal()) {
     // Place holder and final function have to have different IDs, otherwise
     // translation code will reject one of them.

flang/test/Transforms/debug-fn-info.fir

@@ -76,7 +76,7 @@ module attributes {dlti.dl_spec = #dlti.dl_spec<>} {
 // CHECK: #[[TY3:.*]] = #llvm.di_subroutine_type<callingConvention = DW_CC_normal, types = #di_null_type, #[[INT4]]>
 
 // Line numbers should match the number in corresponding loc entry.
-// CHECK: #llvm.di_subprogram<{{.*}}name = "mn", linkageName = "_QQmain", file = {{.*}}, line = 15, scopeLine = 15, subprogramFlags = Definition, type = #[[TY0]]>
+// CHECK: #llvm.di_subprogram<{{.*}}name = "mn", linkageName = "_QQmain", file = {{.*}}, line = 15, scopeLine = 15, subprogramFlags = "Definition|MainSubprogram", type = #[[TY0]]>
 // CHECK: #llvm.di_subprogram<{{.*}}name = "fn1", linkageName = "_QFPfn1", file = {{.*}}, line = 26, scopeLine = 26, subprogramFlags = Definition, type = #[[TY1]]>
 // CHECK: #llvm.di_subprogram<{{.*}}name = "fn2", linkageName = "_QFPfn2", file = {{.*}}, line = 43, scopeLine = 43, subprogramFlags = Definition, type = #[[TY2]]>
 // CHECK: #llvm.di_subprogram<{{.*}}name = "fn3", linkageName = "_QFPfn3", file = {{.*}}, line = 53, scopeLine = 53, subprogramFlags = Definition, type = #[[TY3]]>