When a single #embed directive is used to initialize a char array, the
case is optimized via swap of EmbedExpr to underlying StringLiteral,
resulting in better performance in AST consumers.
While browsing through the code, I realized that
7122b70cfc8e23a069410215c363da76d842bda4
which changed type of EmbedExpr made the "fast path" unreachable. This
patch fixes this unfortunate situation.