[clang][analyzer] Fix the false positive ArgInitializedness warning on unnamed bit-field (#145066)

For the following code in C mode: https://godbolt.org/z/3eo1MeGhe
(There is no warning in C++ mode though).
```c++
struct B {
  int i : 2;
  int : 30;  // unnamed bit-field
};

extern void consume_B(struct B);

void bitfield_B_init(void) {
  struct B b1;
  b1.i = 1; // b1 is initialized
  consume_B(b1); // FP: Passed-by-value struct argument contains uninitialized data (e.g., field: '') [core.CallAndMessage]
}
```

clang/lib/StaticAnalyzer/Checkers/CallAndMessageChecker.cpp

@@ -253,6 +253,8 @@ public:
       const RecordDecl *RD = RT->getDecl()->getDefinition();
       assert(RD && "Referred record has no definition");
       for (const auto *I : RD->fields()) {
+        if (I->isUnnamedBitField())
+          continue;
         const FieldRegion *FR = MrMgr.getFieldRegion(I, R);
         FieldChain.push_back(I);
         T = I->getType();

clang/test/Analysis/call-and-message.c

@@ -1,12 +1,19 @@
 // RUN: %clang_analyze_cc1 %s -verify \
 // RUN:   -analyzer-checker=core \
 // RUN:   -analyzer-config core.CallAndMessage:ArgPointeeInitializedness=true \
+// RUN:   -analyzer-config core.CallAndMessage:ArgInitializedness=false \
 // RUN:   -analyzer-output=plist -o %t.plist
 // RUN: cat %t.plist | FileCheck %s
 
 // RUN: %clang_analyze_cc1 %s -verify=no-pointee \
 // RUN:   -analyzer-checker=core \
-// RUN:   -analyzer-config core.CallAndMessage:ArgPointeeInitializedness=false
+// RUN:   -analyzer-config core.CallAndMessage:ArgPointeeInitializedness=false \
+// RUN:   -analyzer-config core.CallAndMessage:ArgInitializedness=false
+
+// RUN: %clang_analyze_cc1 %s -verify=arg-init \
+// RUN:   -analyzer-checker=core \
+// RUN:   -analyzer-config core.CallAndMessage:ArgPointeeInitializedness=false \
+// RUN:   -analyzer-config core.CallAndMessage:ArgInitializedness=true
 
 // no-pointee-no-diagnostics
 
@@ -22,3 +29,21 @@ void pointee_uninit(void) {
 // checker, as described in the CallAndMessage comments!
 // CHECK: <key>issue_hash_content_of_line_in_context</key>
 // CHECK-SAME: <string>97a74322d64dca40aa57303842c745a1</string>
+
+typedef struct {
+  int i  :2;
+  int    :30;  // unnamed bit-field
+} B;
+
+extern void consume_B(B);
+
+void bitfield_B_init(void) {
+  B b1;
+  b1.i = 1; // b1 is initialized
+  consume_B(b1);
+}
+
+void bitfield_B_uninit(void) {
+  B b2;
+  consume_B(b2); // arg-init-warning{{Passed-by-value struct argument contains uninitialized data (e.g., field: 'i') [core.CallAndMessage]}}
+}

clang/test/Analysis/call-and-message.cpp

@@ -169,4 +169,20 @@ void record_uninit() {
 // CHECK-SAME: <string>a46bb5c1ee44d4611ffeb13f7f499605</string>
 // CHECK: <key>issue_hash_content_of_line_in_context</key>
 // CHECK-SAME: <string>e0e0d30ea5a7b2e3a71e1931fa0768a5</string>
+
+struct B{
+  int i  :2;
+  int    :30;  // unnamed bit-field
+};
+
+void bitfield_B_init(void) {
+  B b1;
+  b1.i = 1; // b1 is initialized
+  consume(b1);
+}
+
+void bitfield_B_uninit(void) {
+  B b2;
+  consume(b2); // arg-init-warning{{Passed-by-value struct argument contains uninitialized data (e.g., field: 'i') [core.CallAndMessage]}}
+}
 } // namespace uninit_arg