Make diagnostics API safer to use

I received a crash report in DiagnosticManager that was caused by a
nullptr diagnostic having been added. The API allows passing in a null
unique_ptr, but all the methods are written assuming that all pointers
a dereferencable. This patch makes it impossible to add a null
diagnostic.

rdar://107633615

Differential Revision: https://reviews.llvm.org/D148823

Author: adrian-prantl

lldb/include/lldb/Expression/DiagnosticManager.h

@@ -114,7 +114,8 @@ class DiagnosticManager {
   }
 
   void AddDiagnostic(std::unique_ptr<Diagnostic> diagnostic) {
-    m_diagnostics.push_back(std::move(diagnostic));
+    if (diagnostic)
+      m_diagnostics.push_back(std::move(diagnostic));
   }
 
   size_t Printf(DiagnosticSeverity severity, const char *format, ...)

lldb/unittests/Expression/DiagnosticManagerTest.cpp

@@ -75,6 +75,9 @@ TEST(DiagnosticManagerTest, HasFixits) {
 TEST(DiagnosticManagerTest, GetStringNoDiags) {
   DiagnosticManager mgr;
   EXPECT_EQ("", mgr.GetString());
+  std::unique_ptr<Diagnostic> empty;
+  mgr.AddDiagnostic(std::move(empty));
+  EXPECT_EQ("", mgr.GetString());
 }
 
 TEST(DiagnosticManagerTest, GetStringBasic) {