Avoid O(n*m) complexity in StringRef::find_first(_not)_of(StringRef).

- Cache used characters in a bitset to reduce memory overhead to just 32 bytes.
- On my core2 this code is faster except when the checked string was very short
  (smaller than the list of delimiters).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@111817 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/StringRef.h

@@ -238,7 +238,7 @@ namespace llvm {
     /// find_first_of - Find the first character in the string that is in \arg
     /// Chars, or npos if not found.
     ///
-    /// Note: O(size() * Chars.size())
+    /// Note: O(size() + Chars.size())
     size_type find_first_of(StringRef Chars, size_t From = 0) const;
 
     /// find_first_not_of - Find the first character in the string that is not
@@ -248,7 +248,7 @@ namespace llvm {
     /// find_first_not_of - Find the first character in the string that is not
     /// in the string \arg Chars, or npos if not found.
     ///
-    /// Note: O(size() * Chars.size())
+    /// Note: O(size() + Chars.size())
     size_type find_first_not_of(StringRef Chars, size_t From = 0) const;
 
     /// @}