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
