Consistent naming for Expression and Value. (#621)

- `Kind` enumerator names always match the corresponding factory function, accessor, and type names (if any).
- All abbreviations in those names are expanded.
- Those names always have a suffix to disambiguate expressions from values.

`VarTV` is excluded from these changes because there's a pending PR to remove it.

executable_semantics/syntax/paren_contents.cpp

@@ -16,7 +16,7 @@ const Expression* ParenContents::AsExpression(int line_number) const {
 }
 
 const Expression* ParenContents::AsTuple(int line_number) const {
-  return Expression::MakeTuple(line_number, fields_);
+  return Expression::MakeTupleLiteral(line_number, fields_);
 }
 
 }  // namespace Carbon