SILGen: Split off SILGenMaterializeForSet.cpp from SILGenLValue.cpp, NFC

In order to fix some other issues that came up with materializeForSet
and resilience, I had to bite the bullet and finish off John's code
for open-coding materializeForSet emission in SILGen. This patch
makes the subsequent changes easier to review.

Author: slavapestov

lib/SILGen/CMakeLists.txt

@@ -19,6 +19,7 @@ add_swift_library(swiftSILGen
   SILGenFunction.cpp
   SILGenGlobalVariable.cpp
   SILGenLValue.cpp
+  SILGenMaterializeForSet.cpp
   SILGenPattern.cpp
   SILGenPoly.cpp
   SILGenProfiling.cpp