[compiler-rt] [test] Allow expanding lit substitutions recursively

[broadwaylamb]

Summary

This allows using lit substitutions in the COMPILER_RT_EMULATOR variable.

The main motivation for this change is using the [ssh.py script from libc++](https://github.com/llvm/llvm-project/blob/master/libcxx/utils/ssh.py) as COMPILER_RT_EMULATOR to copy a test file to the remote machine and execute it there.

This script requires us to pass it the --execdir option with a path that is essentially %T. Then, in my CMake cache file I can just do:

set(COMPILER_RT_EMULATOR "\\\"${Python3_EXECUTABLE}\\\" \\\"${LLVM_PROJECT_DIR}/libcxx/utils/ssh.py\\\" --execdir %%T --host ${REMOTE_TEST_USER}@${REMOTE_TEST_HOST} --" CACHE STRING "")

(For reference, the ability to expand substitutions recursively has been introduced in D76178.)

[compnerd]

Many of the other libraries (unwind, libc++, libc++abi) already configure this similarly. I think that having the same beahviour is probably better. Thanks for fixing this incongruency!

compiler-rt/test/lit.common.cfg.py

 if use_lit_shell:
     # 0 is external, "" is default, and everything else is internal.
     execute_external = (use_lit_shell == "0")
 else:
     # Otherwise we default to internal on Windows and external elsewhere, as
     # bash on Windows is usually very slow.
     execute_external = (not sys.platform in ['win32'])
 
+# Allow expanding substitutions that are based on other substitutions
+config.recursiveExpansionLimit = 10
+
 # Setup test format.
 config.test_format = lit.formats.ShTest(execute_external)
 if execute_external:
   config.available_features.add('shell')
 
 compiler_id = getattr(config, 'compiler_id', None)
 if compiler_id == "Clang":
   if platform.system() != 'Windows':