[libcxx] [test] Rename _Tp to T. NFCI.

This improves readability and (theoretically) improves portability,
as _Ugly names are reserved.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@310758 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/numerics/rand/rand.eng/rand.eng.mers/values.pass.cpp

@@ -42,8 +42,8 @@
 
 #include "test_macros.h"
 
-template <class _Tp>
-void where(const _Tp &) {}
+template <class T>
+void where(const T &) {}
 
 void
 test1()