[tsan] Replace new/delete with a local variable in ThreadSpecificDtors unit test

On OS X, this unit test (ThreadSpecificDtors) fails, because the new and delete operators actually call the overridden operators, which end up using TLVs and crash. Since C++'s new and delete is not important in this test, let's just replace them with a local variable. This fixes the test on OS X.

Differential Revision: http://reviews.llvm.org/D14826

llvm-svn: 253583