[lldb/Reproducers] Implement custom replayers for (char *, size_t)

Some SB API methods returns strings through a char* and a length. This
is a problem for the deserializer, which considers a single type at a
time, and therefore cannot know how many bytes to allocate for the
character buffer.

We can solve this problem by implementing a custom replayer, which
ignores the passed-in char* and allocates a buffer of the correct size
itself, before invoking the original API method or function.

This patch adds three new macros to register a custom replayer for
methods that take a char* and a size_t. It supports arbitrary return
values (some functions return a bool while others return a size_t).