swift234

[REPL] Don't recursively IRGen imported modules if source imports are off.

This was causing all the external definitions to go from SILGen through
IRGen multiple times per REPL statement. Since this only matters if we
have source modules enabled, just turn it off.

We still generate all the external definitions for every REPL /line/, which
is the cause of <rdar://problem/16164076>.

Swift SVN r14467