llvm-objdump: Replace the -macho -raw option with a generic -raw-clang-ast

llvm-objdump: Replace the -macho -raw option with a generic -raw-clang-ast
option that works with all object container formats.
Now that clang modules/PCH are object containers this option is useful to
to construct pipes like

  llvm-objdump -raw-clang-ast foo.pcm | llvm-bcanalyzer -

to inspect the AST contents in a PCH container.
Will be tested via clang.

Belatedly addresses review feedback for r233390.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241659 91177308-0d34-0410-b5e6-96231b3b80d8