PR16074, implement warnings to catch pointer to boolean true and pointer to

PR16074, implement warnings to catch pointer to boolean true and pointer to
null comparison when the pointer is known to be non-null.

This catches the array to pointer decay, function to pointer decay and
address of variables.  This does not catch address of function since this
has been previously used to silence a warning.

Pointer to bool conversion is under -Wbool-conversion.
Pointer to null comparison is under -Wtautological-pointer-compare, a sub-group
of -Wtautological-compare.

void foo() {
  int arr[5];
  int x;
  // warn on these conditionals
  if (foo);
  if (arr);
  if (&x);
  if (foo == null);
  if (arr == null);
  if (&x == null);

  if (&foo);  // no warning
}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202216 91177308-0d34-0410-b5e6-96231b3b80d8