[lldb] Remove the requirement for windows clients to specify -DIMPORT_LIBLLDB

This macro was being used to select the proper import/export annotations
on SB classes. Non-windows clients do not have such requirements.

Instead introduce a new macro (LLDB_IN_LIBLLDB), which signals that
we're compiling liblldb itself (and should use dllexport). The default
(no macro) is to use dllimport. I've moved the macro definition to
SBDefines.h, since it only makes sense when building the API library.

Differential Revision: https://reviews.llvm.org/D117564