Fix building with GCC5 after e64f99c5

It was failing with:

  /work/llvm.monorepo/clang-tools-extra/clangd/ClangdServer.cpp: In lambda function:
  /work/llvm.monorepo/clang-tools-extra/clangd/ClangdServer.cpp:374:75:
  error: could not convert ‘(const char*)""’ from ‘const char*’ to ‘llvm::StringLiteral’
                                                  trace::Metric::Distribution);
                                                                             ^