[lldb] Fix Clang modules build after D101329

D101329 introduces the Process:SaveCore function returning a
`llvm::Expected<bool>`. That function causes that Clang with -fmodules crashes
while compiling LLDB's PythonDataObjects.cpp. With enabled asserts Clang fails
because of:

    Assertion failed: (CachedFieldIndex && "failed to find field in parent")

Crash can be reproduced by building via -DLLVM_ENABLE_MODULES=On with Clang
12.0.1 and then building PythonDataObjects.cpp.o .

Clang bug is tracked at rdar://82901462