[PAuth][LLDB] Support `clang::ptrauth_struct` attribute in user expressions

[kovdan01]

Quote from docs:

... attribute allows programmers to opt in to using signed pointers for all pointers to a particular struct, union, or C++ class type. In C++, this also includes references to the type.

Support for the attribute in lldb user expressions includes the following:

• support dwarf encoding for the attribute
• support parsing of the dwarf-encoded attribute info