RequirementMachine: Introduce 'concrete contraction' pre-processing pass before building rewrite system

See the comment at the top of ConcreteContraction.cpp for a detailed explanation.

This can be turned off with the -disable-requirement-machine-concrete-contraction
pass, mostly meant for testing. A few tests now run with this pass both enabled
and disabled, to exercise code paths which are otherwise trivially avoided by
concrete contraction.

Fixes rdar://problem/88135912.

Author: slavapestov

include/swift/Basic/LangOptions.h

@@ -530,6 +530,11 @@ namespace swift {
     RequirementMachineMode RequirementMachineInferredSignatures =
         RequirementMachineMode::Disabled;
 
+    /// Disable preprocessing pass to eliminate conformance requirements
+    /// on generic parameters which are made concrete. Usually you want this
+    /// enabled. It can be disabled for debugging and testing.
+    bool EnableRequirementMachineConcreteContraction = true;
+
     /// Enables dumping type witness systems from associated type inference.
     bool DumpTypeWitnessSystems = false;
 

include/swift/Option/FrontendOptions.td

@@ -355,6 +355,10 @@ def requirement_machine_max_concrete_nesting : Joined<["-"], "requirement-machin
   Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
   HelpText<"Set the maximum concrete type nesting depth before giving up">;
 
+def disable_requirement_machine_concrete_contraction : Flag<["-"], "disable-requirement-machine-concrete-contraction">,
+  HelpText<"Disable preprocessing pass to eliminate conformance requirements "
+           "on generic parameters which are made concrete">;
+
 def dump_type_witness_systems : Flag<["-"], "dump-type-witness-systems">,
   HelpText<"Enables dumping type witness systems from associated type inference">;
 

lib/AST/CMakeLists.txt

@@ -75,6 +75,7 @@ add_swift_host_library(swiftAST STATIC
   ProtocolConformance.cpp
   RawComment.cpp
   RequirementEnvironment.cpp
+  RequirementMachine/ConcreteContraction.cpp
   RequirementMachine/ConcreteTypeWitness.cpp
   RequirementMachine/GenericSignatureQueries.cpp
   RequirementMachine/HomotopyReduction.cpp