Don't leak the CXStoredDiagnostics returned by clang_codeCompleteGetDiagnostic()

r144269 changed clang_disposeDiagnostic() to be a no-op, but didn't update
code completion diagnostics.  Let CXCodeCompleteResults store all diagnostics
returned by clang_codeCompleteGetDiagnostic() and then free them up in
clang_disposeCodeCompleteResults().

Code completion diagnostics referred to data stored in CXCodeCompleteResults
before already, so it wasn't possible to refer to the results of
clang_codeCompleteGetDiagnostic() after clang_disposeCodeCompleteResults()
before this change already -- hence this should be a safe, backwards-compatible
change.

Leak found by LSan, fixes PR19690.