Adding fixes for signedness warnings in libsolidity

Co-authored-by: Kamil Śliwak <kamil.sliwak@codepoets.it>

libsolidity/ast/AST.cpp

@@ -38,7 +38,7 @@ using namespace solidity;
 using namespace solidity::frontend;
 
 ASTNode::ASTNode(int64_t _id, SourceLocation _location):
-	m_id(_id),
+	m_id(static_cast<size_t>(_id)),
 	m_location(std::move(_location))
 {
 }