Move to clang toolchain

See ghc#21019.
3 jobs for wip/clang in 1 minute and 29 seconds (queued for 9 seconds)
Name Stage Failure
failed
test-i686 Test
+ CC=/c/GitLabRunner/builds/1/989831/tmp/mingw32/bin/clang.exe
+ CFLAGS='-rtlib=compiler-rt -fuse-ld=lld'
++ pwd
+ export 'PATH=/c/GitLabRunner/builds/1/989831/tmp/mingw32/bin:/c/Program Files/Git/cmd:/usr/bin'
+ PATH='/c/GitLabRunner/builds/1/989831/tmp/mingw32/bin:/c/Program Files/Git/cmd:/usr/bin'
+ /c/GitLabRunner/builds/1/989831/tmp/mingw32/bin/clang.exe --version
test.sh: line 32: /c/GitLabRunner/builds/1/989831/tmp/mingw32/bin/clang.exe: No such file or directory
Cleaning up file based variables
ERROR: Job failed: exit status 1