Fix mysterious failure of SupportTests FileCheckTest.Binop

The test would fail in no-asserts release builds using MSVC
for 64-bit Windows:

Unexpected error message:
TestBuffer:1:1: error: implicit format conflict between 'FOO' (%u) and '18\0' (%x), need an explicit format specifier

Error message(s) not found:
{implicit format conflict between 'FOO' (%u) and 'BAZ' (%x), need an explicit format specifier}

It seems a string from a previous test case is finding its way
into the latter one.

This doesn't reproduce on master anymore after 998709b, so let's
just hack around it here for the branch.

llvm/unittests/Support/FileCheckTest.cpp

@@ -714,6 +714,7 @@ TEST_F(FileCheckTest, Binop) {
   Value = Binop.eval();
   expectUndefErrors({"FOO", "BAR"}, Value.takeError());
 
+  {
   // Literal + Variable has format of variable.
   ExprStr = bufferize(SM, "FOO+18");
   FooStr = ExprStr.take_front(3);
@@ -736,6 +737,7 @@ TEST_F(FileCheckTest, Binop) {
   ImplicitFormat = Binop.getImplicitFormat(SM);
   ASSERT_THAT_EXPECTED(ImplicitFormat, Succeeded());
   EXPECT_EQ(*ImplicitFormat, ExpressionFormat::Kind::Unsigned);
+  }
 
   // Variables with different implicit format conflict.
   ExprStr = bufferize(SM, "FOO+BAZ");