[serialized diagnostics]: add test cases for serialized diagnostics, including a test case for no issues, multiple issues, and
a single issue.  Along the way, tweak c-index-test -read-diagnostics output so it is easier to tell what diagnostics are
child diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144349 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed