[llvm-nm] Small fix to Exected<StringRef>
[llvm-nm] Small fix to Exected<StringRef>

Differential Revision: https://reviews.llvm.org/D66976

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@370474 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm