GDScript: Suppress unused constant warning with underscore

The warning message mentions that local constants prefixed with `_` does
not generate the warning. This commit actually implements this warning
suppression.

modules/gdscript/gdscript_analyzer.cpp

@@ -1985,7 +1985,7 @@ void GDScriptAnalyzer::resolve_constant(GDScriptParser::ConstantNode *p_constant
 
 #ifdef DEBUG_ENABLED
 	if (p_is_local) {
-		if (p_constant->usages == 0) {
+		if (p_constant->usages == 0 && !String(p_constant->identifier->name).begins_with("_")) {
 			parser->push_warning(p_constant, GDScriptWarning::UNUSED_LOCAL_CONSTANT, p_constant->identifier->name);
 		}
 	}

modules/gdscript/tests/scripts/parser/features/constants.out

@@ -1,33 +1 @@
 GDTEST_OK
->> WARNING
->> Line: 2
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_TEST" is declared but never used in the block. If this is intended, prefix it with an underscore: "__TEST".
->> WARNING
->> Line: 3
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_STRING" is declared but never used in the block. If this is intended, prefix it with an underscore: "__STRING".
->> WARNING
->> Line: 4
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_VECTOR" is declared but never used in the block. If this is intended, prefix it with an underscore: "__VECTOR".
->> WARNING
->> Line: 5
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_ARRAY" is declared but never used in the block. If this is intended, prefix it with an underscore: "__ARRAY".
->> WARNING
->> Line: 6
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_DICTIONARY" is declared but never used in the block. If this is intended, prefix it with an underscore: "__DICTIONARY".
->> WARNING
->> Line: 9
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_HELLO" is declared but never used in the block. If this is intended, prefix it with an underscore: "__HELLO".
->> WARNING
->> Line: 10
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_INFINITY" is declared but never used in the block. If this is intended, prefix it with an underscore: "__INFINITY".
->> WARNING
->> Line: 11
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_NOT_A_NUMBER" is declared but never used in the block. If this is intended, prefix it with an underscore: "__NOT_A_NUMBER".

modules/gdscript/tests/scripts/parser/features/static_typing.out

@@ -1,21 +1 @@
 GDTEST_OK
->> WARNING
->> Line: 11
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_INTEGER" is declared but never used in the block. If this is intended, prefix it with an underscore: "__INTEGER".
->> WARNING
->> Line: 12
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_INTEGER_REDUNDANT_TYPED" is declared but never used in the block. If this is intended, prefix it with an underscore: "__INTEGER_REDUNDANT_TYPED".
->> WARNING
->> Line: 13
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_INTEGER_REDUNDANT_TYPED2" is declared but never used in the block. If this is intended, prefix it with an underscore: "__INTEGER_REDUNDANT_TYPED2".
->> WARNING
->> Line: 14
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_INTEGER_REDUNDANT_INFERRED" is declared but never used in the block. If this is intended, prefix it with an underscore: "__INTEGER_REDUNDANT_INFERRED".
->> WARNING
->> Line: 15
->> UNUSED_LOCAL_CONSTANT
->> The local constant "_INTEGER_REDUNDANT_INFERRED2" is declared but never used in the block. If this is intended, prefix it with an underscore: "__INTEGER_REDUNDANT_INFERRED2".

modules/gdscript/tests/scripts/parser/warnings/unused_constant.gd

@@ -0,0 +1,4 @@
+func test():
+	const UNUSED = "not used"
+
+	const _UNUSED = "not used, but no warning since the constant name starts with an underscore"

modules/gdscript/tests/scripts/parser/warnings/unused_constant.out

@@ -0,0 +1,5 @@
+GDTEST_OK
+>> WARNING
+>> Line: 2
+>> UNUSED_LOCAL_CONSTANT
+>> The local constant "UNUSED" is declared but never used in the block. If this is intended, prefix it with an underscore: "_UNUSED".