[libcxx] Patch/RFC: Avoid using ::operator new in the default allocator

[Benjamin Kramer]

Since N3664 was implemented in Clang (r186799) it can't optimize unused pairs of ::operator new and ::operator delete anymore. Calls generated by a new/delete expression are still foldable with the updated wording. This affects optimizing away unnecessary code that would be really nice to get right. For example

#include <vector>

int main() {
  std::vector<int> v;
  v.push_back(1);

  return v[0];
}

This should fold down to "return 1;" with no allocations. The example is of course oversimplified but situations like this easily occur in real world code through inlining.

The proposed patch replaces "::operator new(x)" with "new char[x]" and adds the necessary casts in the allocator class, as suggested by Richard Smith. This is sufficient to constant fold code like my test case again.

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- Ben