[StringExtras] Add llvm::to_integer.

This is a very thin wrapper around StringRef::getAsInteger.
It serves three purposes.

1) It allows a cleaner syntax when you have something other than
   a StringRef - for example, a std::string or an llvm::SmallString.
   Previously, in this case you would have to write something like:
      StringRef(MyStr).getAsInteger(0, Result)
   by explicitly constructing a temporary StringRef.  This can be
   done implicitly however with the new function by just writing:
      to_integer(MyStr, ...).
2) Correcting the travesty that is getAsInteger's return value.
   This function returns true on success, and false on failure.
   While this may cause confusion with people familiar with the
   getAsInteger API, there seems to be widespread agreement that
   the return semantics of getAsInteger was a mistake.
3) It allows the Radix to be deduced as a default argument by
   putting it last in the parameter list.  Most uses of getAsInteger
   pass 0 for the first argument.  With this syntax it can just be
   omitted.

llvm-svn: 303011