[test] Reinstate the assignment to the diagnostic log in the unittest

from r363009

The diagnostic log is now set to "-" which forces it to use STDERR
instead of the filesystem. A new comment is added to explain why
the assignment is needed in the test.

llvm-svn: 363199

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/unittests/Frontend/CompilerInstanceTest.cpp b/clang/unittests/Frontend/CompilerInstanceTest.cpp
index 3b3dc527df5..4935853f138 100644
--- a/clang/unittests/Frontend/CompilerInstanceTest.cpp
+++ b/clang/unittests/Frontend/CompilerInstanceTest.cpp
@@ -73,6 +73,10 @@ TEST(CompilerInstance, DefaultVFSOverlayFromInvocation) {
 
 TEST(CompilerInstance, AllowDiagnosticLogWithUnownedDiagnosticConsumer) {
   auto DiagOpts = new DiagnosticOptions();
+  // Tell the diagnostics engine to emit the diagnostic log to STDERR. This
+  // ensures that a chained diagnostic consumer is created so that the test can
+  // exercise the unowned diagnostic consumer in a chained consumer.
+  DiagOpts->DiagnosticLogFile = "-";
 
   // Create the diagnostic engine with unowned consumer.
   std::string DiagnosticOutput;