[VPlan] Move test strs to variables, silence spurious C4129 (NFC).

This is a speculative fix to silence the spurious C4129 warning that
some version of MSVC generate for the raw string literals in the changed
files.

Before disabling the warning (D76428), try a potential fix suggested in
the review.

llvm/unittests/Transforms/Vectorize/VPlanHCFGTest.cpp

@@ -94,7 +94,7 @@ TEST_F(VPlanHCFGTest, testBuildHCFGInnerLoop) {
   Plan->addVPValue(&*F->arg_begin());
   std::string FullDump;
   raw_string_ostream(FullDump) << *Plan;
-  EXPECT_EQ(R"(digraph VPlan {
+  const char *ExpectedStr = R"(digraph VPlan {
 graph [labelloc=t, fontsize=30; label="Vectorization Plan"]
 node [shape=rect, fontname=Courier, fontsize=30]
 edge [fontname=Courier, fontsize=30]
@@ -125,8 +125,8 @@ compound=true
     ]
   }
 }
-)",
-            FullDump);
+)";
+  EXPECT_EQ(ExpectedStr, FullDump);
 
   LoopVectorizationLegality::InductionList Inductions;
   SmallPtrSet<Instruction *, 1> DeadInstructions;

llvm/unittests/Transforms/Vectorize/VPlanTest.cpp

@@ -209,7 +209,7 @@ TEST(VPBasicBlockTest, print) {
   std::string FullDump;
   raw_string_ostream(FullDump) << Plan;
 
-  EXPECT_EQ(R"(digraph VPlan {
+  const char *ExpectedStr = R"(digraph VPlan {
 graph [labelloc=t, fontsize=30; label="Vectorization Plan"]
 node [shape=rect, fontname=Courier, fontsize=30]
 edge [fontname=Courier, fontsize=30]
@@ -227,8 +227,8 @@ compound=true
       "EMIT ret vp<%2>\l"
   ]
 }
-)",
-            FullDump);
+)";
+  EXPECT_EQ(ExpectedStr, FullDump);
 
   {
     std::string I3Dump;