parse.y: Fix the location of a target constant of OP_CDECL
p RubyVM::AbstractSyntaxTree.parse("::Foo += 1").children #=> before: [[], nil, (OP_CDECL@1:0-1:10 (COLON3@1:0-1:10 :Foo) :+ (LIT@1:9-1:10 1))] #=> after: [[], nil, (OP_CDECL@1:0-1:10 (COLON3@1:0-1:5 :Foo) :+ (LIT@1:9-1:10 1))]
parse.y: Fix the location of a target constant of OP_CDECL