swift234

Added SILExternalSource, a class that can be used
to interpose on the SIL linking process.  Under the
following conditions:

- a function is called by the current SILModule;
- that function is transparent and empty in the
  generated SIL;
- the SIL linker doesn't find a serialized
  version of it; and
- the external client (right now, LLDB) has a
  SILExternalSource installed in the SILModule;

the SIL linker will query the SILExternalSource
via lookupSILFunction.

This interface will be used by LLDB to provide
SIL accessors for a program's variables when they
are referred to by Swift expressions.


Swift SVN r9452