To reproduce this, one can run the following LLDB test:

D:\python_src\Python-2.7.8\PCbuild\python_d.exe  D:\src\llvm\tools\lldb\test/dotest.py -q --arch=i686 --executable D:/src/llvm/build/ninja/bin/lldb.exe -s D:/src/llvm/build/ninja/lldb-test-traces -u CXXFLAGS -u CFLAGS -C D:\src\llvm\build\ninja\bin\clang.exe -p TestDataFormatterLibcxxVBool.py D:\src\llvm\tools\lldb\test\functionalities\data-formatter\data-formatter-stl\libcxx\vbool

You will get the following assertion failure:

Assertion failed: Allowed && "Externally-placed field cannot be placed here", file ..\..\tools\clang\lib\AST\RecordLayoutBuilder.cpp, line 1738

This is happening because in RecordLayoutBuilder.cpp, the following check in ASTContext::getASTRecordLayout():

  if (isMsLayout(D) && !D->getASTContext().getExternalSource()) {
    NewEntry = BuildMicrosoftASTRecordLayout(D);
  }

is failing since getExternalSource() is returning true, so it is trying to use non-MSABI record layout.

Data formatter tests need to be disabled on Windows until this is fixed, so please re-enable them, or file a bug to re-enable them, once this is fixed.