[libcxx] [test] Change comments to say C++ instead of c++. NFC.

This makes them consistent (many comments already used uppercase).

The special REQUIRES, UNSUPPORTED, and XFAIL comments are excluded from this change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@309468 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/unord.hash/pointer.pass.cpp

@@ -41,7 +41,7 @@ test()
     assert(h(&i) != h(&j));
 }
 
-// can't hash nullptr_t until c++17
+// can't hash nullptr_t until C++17
 void test_nullptr()
 {
 #if TEST_STD_VER > 14