tsan: introduce New/Alloc/Free helpers

We frequenty allocate sizeof(T) memory and call T ctor on that memory
(C++ new keyword effectively). Currently it's quite verbose and
usually takes 2 lines of code.
Add New<T>() helper that does it much more concisely.

Rename internal_free to Free that also sets the pointer to nullptr.
Shorter and safer.

Rename internal_alloc to Alloc, just shorter.

Reviewed By: vitalybuka, melver

Differential Revision: https://reviews.llvm.org/D107085