stdlib: use fixed-width Builtin.IntXX types for Swift.Int and UInt

Using the unknown-sized Builtin.Word types complicates producing
compile-time overflow diagnostics.  If we don't know the target Word
size, we don't know if there is an overflow.  But SIL optimizer does not
know the size of Word, this is the point of having the Word type in the
first place.

Also, this opens up more possibilities for optimizations.

rdar://17604532

Swift SVN r24788

utils/SwiftIntTypes.py

@@ -33,7 +33,7 @@ class SwiftIntegerType(object):
             'Int' + \
             ('' if is_word else str(bits))
 
-        self.builtin_name = ('Word' if is_word else 'Int' + str(bits))
+        self.builtin_name = 'Int' + str(bits)
 
     def get_opposite_signedness(self):
         return SwiftIntegerType(self.is_word, self.bits, not self.is_signed)
@@ -102,4 +102,4 @@ def all_integer_assignment_operator_names():
     
 def all_integer_or_real_assignment_operator_names():
     return ['=', '*=', '/=', '%=', '+=', '-=']
-    
+