Move ToString into strings.h

This seems like the obvious place for it, plus I'm hacking around at some
changes that would appreciate having it there.

PiperOrigin-RevId: 303373826
diff --git a/src/main/cpp/option_processor.cc b/src/main/cpp/option_processor.cc
index e44af24..69b73a8 100644
--- a/src/main/cpp/option_processor.cc
+++ b/src/main/cpp/option_processor.cc
@@ -642,9 +642,9 @@
   std::vector<std::string> result = {
       "--rc_source=client",
       "--default_override=0:common=--isatty=" +
-          ToString(IsStandardTerminal()),
+          blaze_util::ToString(IsStandardTerminal()),
       "--default_override=0:common=--terminal_columns=" +
-          ToString(GetTerminalColumns())};
+          blaze_util::ToString(GetTerminalColumns())};
   if (IsEmacsTerminal()) {
     result.push_back("--default_override=0:common=--emacs");
   }