[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.

Author: MaskRay

llvm/tools/llvm-debuginfod/llvm-debuginfod.cpp

@@ -15,6 +15,7 @@
 ///
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Debuginfod/Debuginfod.h"
 #include "llvm/Debuginfod/HTTPClient.h"