Fix isIntegerConstantExpr to compare against zero for casts to bool instead of

Fix isIntegerConstantExpr to compare against zero for casts to bool instead of
truncating.  This allows us to compile:
 void foo() {
  static _Bool foo = 4;
}

into:
@foo1 = internal global i8 1
instead of:
@foo1 = internal global i8 4

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@45779 91177308-0d34-0410-b5e6-96231b3b80d8