Fix building with GCC5 after e64f99c

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);
                                                                             ^

clang-tools-extra/clangd/support/Trace.h

@@ -52,7 +52,7 @@ struct Metric {
     Distribution,
   };
   constexpr Metric(llvm::StringLiteral Name, MetricType Type,
-                   llvm::StringLiteral LabelName = "")
+                   llvm::StringLiteral LabelName = llvm::StringLiteral(""))
       : Name(Name), Type(Type), LabelName(LabelName) {}
 
   /// Records a measurement for this metric to active tracer.