[Constraint system] Begin refactoring type variable binding selection.

Split out the code for selecting potential bindings into a separate file
as a first step before refactoring it for improved clarity and ease of
modification.

Author: rudkx

lib/Sema/CMakeLists.txt

@@ -5,6 +5,7 @@ endif()
 
 add_swift_library(swiftSema STATIC
   CSApply.cpp
+  CSBindings.cpp
   CSDiag.cpp
   CSGen.cpp
   CSPropagate.cpp