[clang] Get rid of "%T" expansions

The %T lit expansion expands to a common directory shared between all the tests in the same directory, which is unexpected and unintuitive, and more importantly, it's been a source of subtle race conditions and flaky tests. In https://reviews.llvm.org/D35396, it was agreed that it would be best to simply ban %T and only keep %t, which is unique to each test. When a test needs a temporary directory, it can just create one using mkdir %t.

This patch removes %T in clang.

Differential Revision: https://reviews.llvm.org/D36437

llvm-svn: 310950