[ParseableInterface] Honor -track-system-dependencies when caching

One interesting thing here is that the decision of whether or not to
track system dependencies ends up going into the cache key for a
swiftmodule built from a parseable interface, because it affects that
module's up-to-date check. If we didn't include
-track-system-dependencies in the cache key, we could end up tracking
system dependencies for some modules but not others in the same build.

There's a bit of a bug here where they're /not/ honored if the
top-level invocation isn't tracking /any/ dependencies, but given
how uncommon this flag is in practice that's probably okay for now.

Still TODO: honor this for -build-swiftmodule-from-parseable-interface
as well. That's currently not tracking dependencies at all and it
probably should.

Author: jrose-apple

test/ParseableInterface/ModuleCache/Inputs/mock-sdk/usr/include/SomeCModule.h

@@ -0,0 +1 @@
+extern int x;

test/ParseableInterface/ModuleCache/Inputs/mock-sdk/usr/include/module.modulemap

@@ -0,0 +1,3 @@
+module SomeCModule {
+  header "SomeCModule.h"
+}

test/ParseableInterface/ModuleCache/SystemDependencies.swiftinterface

@@ -0,0 +1,44 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags: -module-name SystemDependencies
+
+// RUN: %empty-directory(%t)
+// RUN: cp -r %S/Inputs/mock-sdk %t/mock-sdk
+
+// RUN: echo 'import SystemDependencies' | %target-swift-frontend -typecheck - -I %S -sdk %t/mock-sdk -module-cache-path %t/MCP -emit-dependencies-path %t/dummy.d
+// RUN: test -f %t/MCP/SystemDependencies*.swiftmodule
+// RUN: %FileCheck -check-prefix NEGATIVE %s < %t/dummy.d
+// RUN: %{python} %S/Inputs/make-old.py %t/MCP/SystemDependencies*.swiftmodule
+
+// Baseline: running the same command again doesn't rebuild the cached module.
+// RUN: echo 'import SystemDependencies' | %target-swift-frontend -typecheck - -I %S -sdk %t/mock-sdk -module-cache-path %t/MCP -emit-dependencies-path %t/dummy.d
+// RUN: %{python} %S/Inputs/check-is-old.py %t/MCP/SystemDependencies*.swiftmodule
+
+// Now try changing the contents.
+// RUN: echo "// size change" >> %t/mock-sdk/usr/include/SomeCModule.h
+// RUN: echo 'import SystemDependencies' | %target-swift-frontend -typecheck - -I %S -sdk %t/mock-sdk -module-cache-path %t/MCP -emit-dependencies-path %t/dummy.d
+// RUN: %{python} %S/Inputs/check-is-old.py %t/MCP/SystemDependencies*.swiftmodule
+
+// Okay, now let's try again with system dependency tracking on.
+// RUN: echo 'import SystemDependencies' | %target-swift-frontend -typecheck - -I %S -sdk %t/mock-sdk -module-cache-path %t/MCP-system -emit-dependencies-path %t/dummy.d -track-system-dependencies
+// RUN: test -f %t/MCP-system/SystemDependencies*.swiftmodule
+// RUN: %FileCheck -check-prefix CHECK %s < %t/dummy.d
+// RUN: %{python} %S/Inputs/make-old.py %t/MCP-system/SystemDependencies*.swiftmodule
+
+// Baseline: running the same command again doesn't rebuild the cached module.
+// RUN: echo 'import SystemDependencies' | %target-swift-frontend -typecheck - -I %S -sdk %t/mock-sdk -module-cache-path %t/MCP-system -emit-dependencies-path %t/dummy.d -track-system-dependencies
+// RUN: %{python} %S/Inputs/check-is-old.py %t/MCP-system/SystemDependencies*.swiftmodule
+
+// Now try changing the contents.
+// RUN: echo "// size change" >> %t/mock-sdk/usr/include/SomeCModule.h
+// RUN: echo 'import SystemDependencies' | %target-swift-frontend -typecheck - -I %S -sdk %t/mock-sdk -module-cache-path %t/MCP-system -emit-dependencies-path %t/dummy.d -track-system-dependencies
+// RUN: %{python} %S/Inputs/check-is-new.py %t/MCP-system/SystemDependencies*.swiftmodule
+
+// Check that it picked a different cache key.
+// RUN: %empty-directory(%t/MCP-combined)
+// RUN: cp -n %t/MCP/SystemDependencies*.swiftmodule %t/MCP-combined
+// RUN: cp -n %t/MCP-system/SystemDependencies*.swiftmodule %t/MCP-combined
+
+// NEGATIVE-NOT: SomeCModule.h
+// CHECK: SomeCModule.h
+
+import SomeCModule