VS2015 had an issue with the deletion of an operator. Since VS2017 is
the minimum version that LLVM uses, we can assume that VS2017+ is in use
(_MSC_VER >= 1910). Clean up the now defunct workaround.
Delete the non-placement operator new/delete in the SILAllocated CRTP base so that the compiler saves us from accidentally allocating and leaking SILInstructions on the main heap instead of the owning SILModule's BPA.
Swift SVN r5468