clover: Use explicit conversion from llvm::StringRef to std::string
Fixes build after llvm 777180a32b61070a10dd330b4f038bf24e916af1
("[ADT] Make StringRef's std::string conversion operator explicit")
CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>