[CIR] [Lowering] care trailing zero for lowering constant array (llvm#976)

Close llvm#975

See the attached test case for example

Author: ChuanqiXu9

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -2281,8 +2281,12 @@ class CIRGlobalOpLowering
     // Initializer is a constant array: convert it to a compatible llvm init.
     if (auto constArr = mlir::dyn_cast<mlir::cir::ConstArrayAttr>(init.value())) {
       if (auto attr = mlir::dyn_cast<mlir::StringAttr>(constArr.getElts())) {
-        init = rewriter.getStringAttr(attr.getValue());
-      } else if (auto attr = mlir::dyn_cast<mlir::ArrayAttr>(constArr.getElts())) {
+        llvm::SmallString<256> literal(attr.getValue());
+        if (constArr.getTrailingZerosNum())
+          literal.append(constArr.getTrailingZerosNum(), '\0');
+        init = rewriter.getStringAttr(literal);
+      } else if (auto attr =
+                     mlir::dyn_cast<mlir::ArrayAttr>(constArr.getElts())) {
         // Failed to use a compact attribute as an initializer:
         // initialize elements individually.
         if (!(init = lowerConstArrayAttr(constArr, getTypeConverter()))) {

clang/test/CIR/Lowering/str.c

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s -check-prefix=LLVM
+
+void f(char *fmt, ...);
+void test() {
+    f("test\0");
+}
+
+// LLVM: @.str = {{.*}}[6 x i8] c"test\00\00"