[analyzer] Print a diagnostic note even if the region cannot be printed.

[analyzer] Print a diagnostic note even if the region cannot be printed.

There are few cases where we can track the region, but cannot print the note,
which makes the testing limited. (Though, I’ve tested this manually by making
all regions non-printable.) Even though the applicability is limited now, the enhancement
will be more relevant as we start tracking more regions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179396 91177308-0d34-0410-b5e6-96231b3b80d8