[llvm] Document "%T" as deprecated in TestingGuide.rst

[Lekensteyn]

One small comment, LGTM otherwise.

docs/TestingGuide.rst

464

What about suggesting an alternative like:

Use ``rm -rf %t && mkdir %t`` instead if a temporary directory is necessary.

Perhaps that could even replace the Shouldn't be used, because [reason] sentence if brevity is desired.

[jdenny]

%T should also be deprecated (or removed) below, where it's documented incorrectly:

https://llvm.org/docs/CommandGuide/lit.html#pre-defined-substitutions

[kubamracek]

%T should also be deprecated (or removed) below, where it's documented incorrectly:
https://llvm.org/docs/CommandGuide/lit.html#pre-defined-substitutions

Good catch. https://reviews.llvm.org/D48842.

docs/TestingGuide.rst

``%t``
    File path to a temporary file name that could be used for this test case.
    The file name won't conflict with other test cases. You can append to it
    if you need multiple temporaries. This is useful as the destination of
    some redirected output.
 
    Example: ``/home/user/llvm.build/test/MC/ELF/Output/foo_test.s.tmp``
 
 ``%T``
-   Directory of ``%t``.
+   Directory of ``%t``. Deprecated. Shouldn't be used, because it can be easily
+   misused and cause race conditions between tests. 
 
    Example: ``/home/user/llvm.build/test/MC/ELF/Output``
 
 ``%{pathsep}``
 
    Expands to the path separator, i.e. ``:`` (or ``;`` on Windows).
 
 ``%/s, %/S, %/t, %/T:``