Test_cel: Fails when DONT_OPTIMIZE is off

A bug in GCC causes TEST_CEL to return failure under the following
conditions:
1. TEST_FRAMEWORK on
2. DONT_OPTIMIZE off
3. Fedora and Ubuntu
4. GCC 8.2.1
5. Test name: test_cel_dial_pickup
6. There must exist a certain combination of multithreading.
The bug affects arithmetic calculations when the optimization level
is bigger than O1 and the -fpartial-inline flag is on. Provided these
conditions, function ast_str_to_lower() fails to convert to lower case
due to said function being of type force_inline.  The solution is to
remove the "force_inline" type declaration from function ast_str_to_lower()

Change-Id: Ied32e0071f12ed9d5f3b4cdd878b2532a1c769d7

include/asterisk/strings.h

@@ -1212,19 +1212,7 @@ static force_inline int attribute_pure ast_str_case_hash(const char *str)
  *
  * \retval str for convenience
  */
-static force_inline char *attribute_pure ast_str_to_lower(char *str)
-{
-	char *str_orig = str;
-	if (!str) {
-		return str;
-	}
-
-	for (; *str; ++str) {
-		*str = tolower(*str);
-	}
-
-	return str_orig;
-}
+char *attribute_pure ast_str_to_lower(char *str);
 
 /*!
  * \brief Convert a string to all upper-case

main/strings.c

@@ -421,3 +421,18 @@ int ast_vector_string_split(struct ast_vector_string *dest,
 
 	return 0;
 }
+
+char *attribute_pure ast_str_to_lower(char *str)
+{
+	char *str_orig = str;
+	if (!str) {
+		return str;
+	}
+
+	for (; *str; ++str) {
+		*str = tolower(*str);
+	}
+
+	return str_orig;
+}
+