[llvm-debuginfod] Include llvm/Support/StringExtras.h after D155178

To fix undefined errors like to_float. This tool is often not built as
LLVM_ENABLE_HTTPLIB defaults to off (and the external dependency
cpp-httplib is difficult to set up due to a dependency on brotli) and
LLVM_TOOL_LLVM_DEBUGINFOD_BUILD disabling logic in D147185.