[ADT] 'PointerUnion::is' returns 'bool'

Summary:
The return type of 'PointerUnion::is' has been 'int' since it was first
added in March 2009, in SVN r67987, or
https://github.com/llvm/llvm-project/commit/a9c6de15fb3.

The only other change to this member function was a clang-format applied
in December 2015, in SVN r256513, or
https://github.com/llvm/llvm-project/commit/548a49aacc0.

However, since the return value is the result of a `==` comparison, an
implicit cast must be made converting the boolean result to an `int`.
Change the return type to `bool` to remove the need for such a cast.

Test Plan:
I ran llvm-project `check-all` under ASAN, no failures were reported
(other than obviously unrelated tests that were already failing in
ASAN buildbots).

Reviewers: gribozavr, gribozavr2, rsmith, bkramer, dblaikie

Subscribers: dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73836