[CFG] Add a test for a flaky crash in CFGBlock::getLastCondition().

Push the test separately ahead of time in order to find out whether
our Memory Sanitizer bots will be able to find the problem.

If not, I'll add a much more expensive test that repeats the current
test multiple times in order to show up on normal buildbots.
I really apologize for the potential temporary inconvenience!
I'll commit the fix as soon as I get the signal.

Differential Revision: https://reviews.llvm.org/D69962

Author: haoNoQ

clang/test/Analysis/a_flaky_crash.cpp

@@ -0,0 +1,15 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core -verify %s
+
+struct S {
+  S();
+  ~S();
+};
+
+bool bar(S);
+
+// no-crash during diagnostic construction.
+void foo() {
+  int x;
+  if (true && bar(S()))
+    ++x; // expected-warning{{The expression is an uninitialized value. The computed value will also be garbage}}
+}