[libcxx] Update DoNotOptimize variant to accept L and R value variables.

As part of the libcxx tests, there is a function provided called `DoNotOptimize`.
The tests currently utilise a version which only uses the value as an input,
leading to the compiler not removing information relating to the data-flow for
the value. This leads to issues when comparing the pointer values, where one is a
global pointer and the other a pointer which is local to the function being
executed.

Another version of `DoNotOptimize` is available which allows for the value
to be an input and output of the inline assembly, which allows for the
pointers to be successfully compared to one another. This function however
only accepts an L Value reference, where the values are initialized as R
value reference, so use the variant of `DoNotOptmize` described in the first
paragraph.

To enable the use of the second variant in the tests, the type of reference
has been updated so it allows both R and L value references as the input
value. This enables the use of the correct version of the function for the
tests.

Author: Stylie777

libcxx/test/support/test_macros.h

@@ -296,7 +296,7 @@ inline Tp const& DoNotOptimize(Tp const& value) {
 }
 
 template <class Tp>
-inline Tp& DoNotOptimize(Tp& value) {
+inline Tp& DoNotOptimize(Tp&& value) {
 #if defined(__clang__)
   asm volatile("" : "+r,m"(value) : : "memory");
 #else