cgo: add support for enum types

Enum types are implemented as named types (with possible accompanying
typedefs as type aliases). The constants inside the enums are treated as
Go constants like in the Go toolchain.

cgo/libclang_stubs.c

@@ -56,3 +56,11 @@ CXSourceRange tinygo_clang_getCursorExtent(CXCursor c) {
 CXTranslationUnit tinygo_clang_Cursor_getTranslationUnit(CXCursor c) {
 	return clang_Cursor_getTranslationUnit(c);
 }
+
+long long tinygo_clang_getEnumConstantDeclValue(CXCursor c) {
+	return clang_getEnumConstantDeclValue(c);
+}
+
+CXType tinygo_clang_getEnumDeclIntegerType(CXCursor c) {
+	return clang_getEnumDeclIntegerType(c);
+}