Add default .value field to RegExpLiteral.
Add default .value field to RegExpLiteral.

Since RegExpLiteral inherits from the Literal type, it's supposed to have a
"value" field, but since the RegExpLiteral represets itself with
{pattern,flags} fields instead, we have to synthesize a "value" field.

This fixes a few more Babel whole-program validation tests.

benjamn/ast-types,benjamn/ast-types,benjamn/ast-types