build: compiler warning silencing tidy-ups

- tool_getparam: revert an unnecessary/no-op C89 warning silencer.
  Follow-up to 09c9afdd71 #20363

- tool_writeout: add comment saying silencing is a no-op for llvm/clang.
  For `strftime()` it is a GCC-specific, as of llvm/clang v22.1.0.
  Follow-up to f07a98ae11 #20366

- unit1652: drop always-false `!defined(__clang__)` guard.
  Pointed-out-by: Orgad Shaneh
  Ref: #20902
  Follow-up to 7e814c8717 #16062

- unit1652: document that `-Wformat` is necessary for GCC v5 to v8.
  Follow-up to 71cf0d1fca #14772

Closes #20908

src/tool_getparam.c

@@ -3057,7 +3057,8 @@ ParameterError parse_args(int argc, argv_item_t argv[])
   ParameterError err = PARAM_OK;
   struct OperationConfig *config = global->first;
 
-  for(i = 1, stillflags = TRUE; i < argc && !err; i++) {
+  stillflags = TRUE;
+  for(i = 1; i < argc && !err; i++) {
     orig_opt = convert_tchar_to_UTF8(argv[i]);
     if(!orig_opt)
       return PARAM_NO_MEM;

src/tool_writeout.c

@@ -580,7 +580,7 @@ static const char *outtime(const char *ptr, /* %time{ ... */
     if(!result) {
       struct tm utc;
       result = curlx_gmtime(secs, &utc);
-#ifdef __GNUC__
+#ifdef __GNUC__  /* includes llvm/clang, but not affected as of v22.1.0 */
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wformat-nonliteral"
 #endif

tests/unit/unit1652.c

@@ -88,9 +88,9 @@ static CURLcode test_unit1652(const char *arg)
 
   UNITTEST_BEGIN(t1652_setup(&easy))
 
-#if defined(CURL_GNUC_DIAG) && !defined(__clang__)
+#ifdef CURL_GNUC_DIAG
 #pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wformat"
+#pragma GCC diagnostic ignored "-Wformat"  /* for GCC v5 to v8 */
 #pragma GCC diagnostic ignored "-Wformat-zero-length"
 #if __GNUC__ >= 7
 #pragma GCC diagnostic ignored "-Wformat-overflow"
@@ -157,7 +157,7 @@ static CURLcode test_unit1652(const char *arg)
   fail_unless(output[sizeof(output) - 1] == '\0',
               "Truncation of infof input 3");
 
-#if defined(CURL_GNUC_DIAG) && !defined(__clang__)
+#ifdef CURL_GNUC_DIAG
 #pragma GCC diagnostic pop
 #endif