Convert isIntegerConstantExpr to use ASTContext::MakeIntValue.

 - This idiom ensures that the result will have the right width and
   type.

 - Tested on most of x86_64/llvm-test to satisfy my paranoia.

 - This fixes at least the following bugs:
   o UnaryTypeTraitExpr wasn't setting the width correctly.
   o Arithmetic on _Bool wasn't setting the width correctly.

   And probably a number more.

llvm-svn: 64864