Rename three cxx files in unittests to cpp.

LLVM uses cpp as its C++ file extension, these are the only three cxx file in
the monorepo. These files apparently were called to escape a CMake check -- use
the LLVM_OPTIONAL_SOURCES mechanism that's meant as an escape for this case
instead.

No intended behavior change.
https://reviews.llvm.org/D46843

llvm-svn: 332368
diff --git a/llvm/unittests/Support/DynamicLibrary/PipSqueak.cpp b/llvm/unittests/Support/DynamicLibrary/PipSqueak.cpp
new file mode 100644
index 0000000..e2f1cf7
--- /dev/null
+++ b/llvm/unittests/Support/DynamicLibrary/PipSqueak.cpp
@@ -0,0 +1,49 @@
+//===- llvm/unittest/Support/DynamicLibrary/PipSqueak.cpp -----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "PipSqueak.h"
+
+struct Global {
+  std::string *Str;
+  std::vector<std::string> *Vec;
+  Global() : Str(nullptr), Vec(nullptr) {}
+  ~Global() {
+    if (Str) {
+      if (Vec)
+        Vec->push_back(*Str);
+      *Str = "Global::~Global";
+    }
+  }
+};
+
+static Global Glb;
+
+struct Local {
+  std::string &Str;
+  Local(std::string &S) : Str(S) {
+    Str = "Local::Local";
+    if (Glb.Str && !Glb.Str->empty())
+      Str += std::string("(") + *Glb.Str + std::string(")");
+  }
+  ~Local() { Str = "Local::~Local"; }
+};
+
+
+extern "C" PIPSQUEAK_EXPORT void SetStrings(std::string &GStr,
+                                            std::string &LStr) {
+  Glb.Str = &GStr;
+  static Local Lcl(LStr);
+}
+
+extern "C" PIPSQUEAK_EXPORT void TestOrder(std::vector<std::string> &V) {
+  Glb.Vec = &V;
+}
+
+#define PIPSQUEAK_TESTA_RETURN "LibCall"
+#include "ExportedFuncs.cpp"