[compiler-rt] Get rid of "%T" expansions

Summary:
Original patch by Kuba Mracek

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 compiler-rt.

Differential Revision: https://reviews.llvm.org/D48618

llvm-svn: 336661