[Sema|lit] Intro lit substitution for generating the 2 swiftinterfaces

Intro the lit substitution
`%target-swift-emit-module-interfaces(Main.swiftinterface,
Main.private.swiftinterface` to generate both the public and private
swiftinterface. And use it in spi-only-import-exportability.swift.

To verify the generated interfaces I would recommend to keep two calls
to `%target-swift-typecheck-module-from-interface` as each is a
different invocation likely to fail.

Author: xymus

docs/Testing.md

@@ -344,6 +344,12 @@ code for the target that is not the build machine:
   swiftinterface to the given path and passing additional default flags
   appropriate for resilient frameworks.
 
+* ``%target-swift-emit-module-interfaces(`` *swift interface path*,
+  *swift private interface path* ``)`` *other arguments*:
+  run ``swift-frontend`` for the target, emitting both swiftinterfaces
+  to the given paths and passing additional default flags appropriate for
+  resilient frameworks.
+
 * ``%target-swift-typecheck-module-from-interface(`` *swift interface path*
   ``)`` *other arguments*: run ``swift-frontend`` for the target, verifying
   the swiftinterface at the given path and passing additional default flags

test/SPI/spi-only-import-exportability.swift

@@ -20,10 +20,11 @@
 // RUN:   -enable-library-evolution
 
 /// Generate the swiftinterface of the working code and verify it.
-// RUN: %target-swift-emit-module-interface(%t/Client.swiftinterface) \
+// RUN: %target-swift-emit-module-interfaces(%t/Client.swiftinterface, %t/Client.private.swiftinterface) \
 // RUN:   %t/Client.swift -I %t -DSKIP_ERRORS \
 // RUN:   -experimental-spi-only-imports
 // RUN: %target-swift-typecheck-module-from-interface(%t/Client.swiftinterface) -I %t
+// RUN: %target-swift-typecheck-module-from-interface(%t/Client.private.swiftinterface) -I %t
 
 
 //--- PublicLib.swift

test/lit.cfg

@@ -2300,6 +2300,8 @@ config.substitutions.append(('%target-pic-opt', config.target_pic_opt))
 config.substitutions.append(('%target-msvc-runtime-opt',
                              config.target_msvc_runtime_opt))
 
+config.substitutions.append(('%target-swift-emit-module-interfaces\(([^)]+),\w*([^)]+)\)',
+                             SubstituteCaptures(r'%target-swift-emit-module-interface(\1) -emit-private-module-interface-path \2')))
 config.substitutions.append(('%target-swift-emit-module-interface\(([^)]+)\)',
                              SubstituteCaptures(r'%target-swift-frontend -swift-version 5 -enable-library-evolution -typecheck -emit-module-interface-path \1')))