[compiler-rt] [test] Use the parent process env as base env in tests

[broadwaylamb]

Summary

Right now the lit config builds up an environment that the tests will be run in. However, it does it from scratch instead of adding new variables to the parent process environment. This may (and does) result in strange behavior when running tests with an executor (i. e. with the COMPILER_RT_EMULATOR CMake variable set to something).

For example, I'm 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. Under the hood, that script executes the ssh command, which needs some environment variables set properly. Before this patch, the ssh command always returned the 255 exit code with no output whatsoever.

Here this is fixed.

[MaskRay]

Hi, your git commit contains extra Phabricator tags. You can drop Reviewers: Subscribers: Tags: and the text Summary: from the git commit with the following script:

arcfilter () {
        arc amend
        git log -1 --pretty=%B | awk '/Reviewers:|Subscribers:/{p=1} /Reviewed By:|Differential Revision:/{p=0} !p && !/^Summary:$/ {sub(/^Summary: /,"");print}' | git commit --amend --date=now -F -
}

Reviewed By: is considered important by some people. Please keep the tag. (--date=now is my personal preference (author dates are usually not useful. Using committer dates can make log almost monotonic in time))

llvm/utils/git/pre-push.py can validate the message does not include unneeded tags.

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

if config.android:
   config.target_cflags = "-pie -fuse-ld=gold " + config.target_cflags
   if config.android_ndk_version < 19:
     # With a new compiler and NDK < r19 this flag ends up meaning "link against
     # libc++", but NDK r19 makes this mean "link against the stub libstdc++ that
     # just contains a handful of ABI functions", which makes most C++ code fail
     # to link. In r19 and later we just use the default which is libc++.
     config.cxx_mode_flags.append('-stdlib=libstdc++')
 
+config.environment = dict(os.environ)
+
 # Clear some environment variables that might affect Clang.
 possibly_dangerous_env_vars = ['ASAN_OPTIONS', 'DFSAN_OPTIONS', 'LSAN_OPTIONS',
                                'MSAN_OPTIONS', 'UBSAN_OPTIONS',
                                'COMPILER_PATH', 'RC_DEBUG_OPTIONS',
                                'CINDEXTEST_PREAMBLE_FILE', 'LIBRARY_PATH',
                                'CPATH', 'C_INCLUDE_PATH', 'CPLUS_INCLUDE_PATH',
                                'OBJC_INCLUDE_PATH', 'OBJCPLUS_INCLUDE_PATH',
                                'LIBCLANG_TIMING', 'LIBCLANG_OBJTRACKING',