Implement p0292r2 (constexpr if), a likely C++1z feature.

llvm-svn: 273602
diff --git a/clang/test/CodeGenCXX/cxx1z-constexpr-if.cpp b/clang/test/CodeGenCXX/cxx1z-constexpr-if.cpp
new file mode 100644
index 0000000..80a397f
--- /dev/null
+++ b/clang/test/CodeGenCXX/cxx1z-constexpr-if.cpp
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -std=c++1z %s -emit-llvm -o - | FileCheck %s --implicit-check-not=should_not_be_used
+
+void should_be_used_1();
+void should_be_used_2();
+void should_not_be_used();
+void f() {
+  if constexpr (false)
+    should_not_be_used();
+  else
+    should_be_used_1();
+
+  if constexpr (true || ({ label: false; }))
+    should_be_used_2();
+  else {
+    goto foo;
+foo: should_not_be_used();
+  }
+}
+
+// CHECK: should_be_used_1
+// CHECK: should_be_used_2