[flang] Lower PAUSE statement with code and message operands

This patch adds lowering support for the Fortran PAUSE statement to FIR.
It mirrors the STOP statement lowering logic to handle:
  - PAUSE with no operand
  - PAUSE <integer>
  - PAUSE 'message'

  Fixes #166821.

Author: sathvikreddy853

flang/lib/Lower/Runtime.cpp

@@ -263,12 +263,52 @@ void Fortran::lower::genSyncTeamStatement(
 
 void Fortran::lower::genPauseStatement(
     Fortran::lower::AbstractConverter &converter,
-    const Fortran::parser::PauseStmt &) {
+    const Fortran::parser::PauseStmt &stmt) {
   fir::FirOpBuilder &builder = converter.getFirOpBuilder();
   mlir::Location loc = converter.getCurrentLocation();
-  mlir::func::FuncOp callee =
-      fir::runtime::getRuntimeFunc<mkRTKey(PauseStatement)>(loc, builder);
-  fir::CallOp::create(builder, loc, callee, mlir::ValueRange{});
+  Fortran::lower::StatementContext stmtCtx;
+  llvm::SmallVector<mlir::Value> operands;
+  mlir::func::FuncOp callee;
+  mlir::FunctionType calleeType;
+
+  if (stmt.v.has_value()) {
+    const Fortran::parser::StopCode &code = stmt.v.value();
+    auto expr = converter.genExprValue(*Fortran::semantics::GetExpr(code), stmtCtx);
+    LLVM_DEBUG(llvm::dbgs() << "pause expression: "; expr.dump(); llvm::dbgs() << '\n');
+    expr.match(
+        [&](const fir::CharBoxValue &x) {
+          callee = fir::runtime::getRuntimeFunc<mkRTKey(PauseStatementText)>(loc, builder);
+          calleeType = callee.getFunctionType();
+          operands.push_back(
+              builder.createConvert(loc, calleeType.getInput(0), x.getAddr()));
+          operands.push_back(
+              builder.createConvert(loc, calleeType.getInput(1), x.getLen()));
+        },
+        [&](fir::UnboxedValue x) {
+          callee = fir::runtime::getRuntimeFunc<mkRTKey(PauseStatement)>(loc, builder);
+          calleeType = callee.getFunctionType();
+          mlir::Value cast =
+              builder.createConvert(loc, calleeType.getInput(0), x);
+          operands.push_back(cast);
+        },
+        [&](auto) {
+          mlir::emitError(loc, "unhandled expression in PAUSE");
+          std::exit(1);
+        });
+  } else {
+    callee = fir::runtime::getRuntimeFunc<mkRTKey(PauseStatement)>(loc, builder);
+    calleeType = callee.getFunctionType();
+  }
+
+  fir::CallOp::create(builder, loc, callee, operands);
+
+  auto blockIsUnterminated = [&builder]() {
+    mlir::Block *currentBlock = builder.getBlock();
+    return currentBlock->empty() ||
+           !currentBlock->back().hasTrait<mlir::OpTrait::IsTerminator>();
+  };
+  if (blockIsUnterminated())
+    genUnreachable(builder, loc);
 }
 
 void Fortran::lower::genPointerAssociate(fir::FirOpBuilder &builder,