FAILED: obj/native_client/src/trusted/service_runtime/sel/nacl_syscall_common.obj 
../../third_party/llvm-build/Release+Asserts/bin/clang-cl.exe /nologo /showIncludes /FC @obj/native_client/src/trusted/service_runtime/sel/nacl_syscall_common.obj.rsp /c ../../native_client/src/trusted/service_runtime/nacl_syscall_common.c /Foobj/native_client/src/trusted/service_runtime/sel/nacl_syscall_common.obj /Fd"obj/native_client/src/trusted/service_runtime/sel_c.pdb"
Assertion failed: isValidGCCRegisterName(Name) && "Invalid register passed in", file C:\b\c\b\ClangToTWin\src\third_party\llvm\tools\clang\lib\Basic\TargetInfo.cpp, line 415
Wrote crash dump file "C:\Users\CHROME~1\AppData\Local\Temp\clang-cl.exe-d96afc.dmp"
#0 0x0000000140b566c5 (C:\b\c\b\ClangToTWin\src\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe+0x12866c5)
#1 0x00000001422af503 (C:\b\c\b\ClangToTWin\src\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe+0x29df503)
#2 0x000000014229cb00 (C:\b\c\b\ClangToTWin\src\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe+0x29ccb00)
#3 0x000000014229c22e (C:\b\c\b\ClangToTWin\src\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe+0x29cc22e)
#4 0x000000014229c2ca (C:\b\c\b\ClangToTWin\src\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe+0x29cc2ca)
#5 0x00000001423867cc (C:\b\c\b\ClangToTWin\src\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe+0x2ab67cc)
#6 0x0000000140eedcc6 (C:\b\c\b\ClangToTWin\src\third_party\llvm-build\Release+Asserts\bin\clang-cl.exe+0x161dcc6)

Started e.g. here: 
https://build.chromium.org/p/chromium.fyi/builders/ClangToTWin/builds/11403 (clang rev 295016; prev build had 294995).