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>

src/gallium/state_trackers/clover/llvm/codegen/common.cpp

@@ -196,8 +196,9 @@ clover::llvm::build_module_common(const Module &mod,
                                   const clang::CompilerInstance &c) {
    module m;
 
-   for (const auto &name : map(std::mem_fn(&Function::getName),
+   for (const auto &llvm_name : map(std::mem_fn(&Function::getName),
                                get_kernels(mod))) {
+      const ::std::string name(llvm_name);
       if (offsets.count(name))
          m.syms.emplace_back(name, 0, offsets.at(name),
                              make_kernel_args(mod, name, c));

src/gallium/state_trackers/clover/llvm/metadata.hpp

@@ -62,7 +62,7 @@ namespace clover {
                             const std::string &name) {
          return ::llvm::cast< ::llvm::MDString>(
                detail::get_kernel_metadata_operands(f, name)[arg.getArgNo()])
-            ->getString();
+            ->getString().str();
       }
 
       ///