[transfer-nonsendable] Rename tests from sendnonsendable_* -> transfernonsendable_*

I also renamed sendnonsendable_basic.sil ->
transfernonsendable_instruction_matching.sil since that is what the test is
intended to be used for... mechanical instruction level modeling tests rather
than more complex crashers.

Author: gottesmm

test/Concurrency/sendnonsendable_basic.swift renamed to test/Concurrency/transfernonsendable.swift

@@ -1,5 +1,14 @@
 // RUN: %target-sil-opt -transfer-non-sendable -enable-experimental-feature RegionBasedIsolation -strict-concurrency=complete %s -verify
 
+// PLEASE READ THIS!
+//
+// This test is specifically to test out individual instruction interactions,
+// not for crashers. The idea is to make it so that we have one test for every
+// SIL instruction, so please add a test here when you add a new instruction.
+//
+// For assign/lookthrough, just add a test that triggers an ownership error. If it is
+// a more complex instruction, talk with @gottesmm.
+
 sil_stage raw
 
 import Swift
@@ -288,4 +297,4 @@ bb0(%0 : $*NonSendableStruct):
   // expected-warning @-1 {{call site passes `self` or a non-sendable argument of this function to another thread, potentially yielding a race with the caller}}
   %9999 = tuple ()
   return %9999 : $()
-}
+}