[StringRef] Add enable-if to StringLiteral.

to prevent StringLiteral from being created with a non-literal
char array, clang has a macro enable_if() that can be used
in such a way as to guarantee that the constructor is disabled
unless the length fo the string can be computed at compile time.

This only works on clang, but at least it should allow bots
to catch abuse of StringLiteral.

Differential Revision: https://reviews.llvm.org/D27780

llvm-svn: 289853