[TableGen] emitStringLiteralDef: Pad trailing '\0' at the end of char array.

Fixup for https://reviews.llvm.org/D73044
String literal has an implicit terminator '\0'. This commit adjusts char array
to long literal.

This causes difference of artifacts between -long-string-literals=true
and false.

Differential Revision: https://reviews.llvm.org/D126136