Compile-fix after StringRef's conversion operator has been made explicit.

Commit 777180a "[ADT] Make StringRef's std::string conversion operator explicit"
caused Polly's GPU code generator to not compile anymore. The rest of
Polly has already been fixed in commit
0257a9 "Fix polly build after StringRef change."

GitOrigin-RevId: 1a53b732e6bf1bbadd8d352573172c9d0c8b0cca
2 files changed