[llvm-cov] Attempt to appease an older builder
[llvm-cov] Attempt to appease an older builder

It's using a version of clang which can't (or won't) deduce an implicit
conversion from a SmallString to a StringRef. Write the conversion out
explicitly:

  http://lab.llvm.org:8011/builders/lldb-x86_64-ubuntu-14.04-buildserver/builds/8574

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@275647 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm