[uninit] Revise translation of conditional expression for C struct

Summary:
This diff revises the translation of the conditional expression for C struct.

```
struct S = {
  int f1;
  int f2;
}

struct S foo(int b) {
  ...
  return b ? x : y;
}
```

Before this diff, the return statements were translated to

```
n$0 = *&__return_param;
...
*n$0 = x; // at the true branch
...
*n$0 = y; // at the false branch
```

However, this is incorrect because there is no first-value for C struct in SIL.

By this diff, it is translated to multiple instructions that copy each field separately.

```
n$0 = *&__return_param;
...
n$1 =  *&x.f1; // at the true branch
*n$0.f1 = n$1;
n$2 =  *&x.f2;
*n$0.f2 = n$2;
...
n$3 =  *&y.f1; // at the false branch
*n$0.f1 = n$3;
n$4 =  *&y.f2;
*n$0.f2 = n$4;
```

Reviewed By: jvillard

Differential Revision: D49829058

fbshipit-source-id: 3b9dd95d82e0d4e37c709126117ef7c5a130c3d7

infer/src/clang/cTrans.ml

@@ -2070,7 +2070,12 @@ module CTrans_funct (F : CModule_type.CFrontend) : CModule_type.CTranslation = s
                   exp
             in
             let set_temp_var =
-              [Sil.Store {e1= exp_to_init; typ= var_typ; e2= init_exp; loc= sil_loc}]
+              match var_typ with
+              | {Typ.desc= Tstruct struct_name} ->
+                  CStructUtils.struct_copy context.CContext.tenv sil_loc exp_to_init init_exp
+                    ~typ:var_typ ~struct_name
+              | _ ->
+                  [Sil.Store {e1= exp_to_init; typ= var_typ; e2= init_exp; loc= sil_loc}]
             in
             let return = (exp_to_init, var_typ) in
             let tmp_var_res_trans =

infer/tests/codetoanalyze/c/pulse/uninit.c

@@ -213,3 +213,16 @@ void FP_uninit_var_not_read_interproc_ok() {
   int range_var;
   check_range_wrapper(range_var_valid, &range_var);
 }
+
+struct uninit_s global_uninit_s1;
+
+struct uninit_s global_uninit_s2;
+
+struct uninit_s init_by_conditional_exp(int b) {
+  return b ? global_uninit_s1 : global_uninit_s2;
+}
+
+int call_init_by_conditionaql_exp_ok(int b) {
+  struct uninit_s x = init_by_conditional_exp(b);
+  return x.f1;
+}