[lldb] Add an LLDB_DEPRECATED macro similar to LLVM_DEPRECATED

Add an LLDB_DEPRECATED macro similar to LLVM_DEPRECATED. We cannot
directly reuse the LLVM one, because it's defined in Compiler.h which is
not something we can include in the SB API.

For the SB API, the macro is undef'd if:

  1. We're building the SWIG bindings. We might still want to expose
     deprecated methods to our scripting users.

  2. We're targeting anything older than C++14 as the [[deprecated]]
     attribute was added for C++14 and the SB API can be used by
     projects compiling against an older language version.

Differential revision: https://reviews.llvm.org/D147736