[Sema] Accept and store the IPI library-level

Prepare to accept the `ipi` argument to the `-library-level` flag. IPI
stands for Internal Programming Interface and would describe a module
that's not to be distributed outside of its project.

In the future, the compiler could use that information to report when a
distributed module (api or spi) imports publicly a module that's not
distributed.

rdar://102435183

Author: xymus

include/swift/AST/DiagnosticsFrontend.def

@@ -46,7 +46,7 @@ ERROR(error_upcoming_feature_on_by_default, none,
 
 ERROR(error_unknown_library_level, none,
       "unknown library level '%0', "
-      "expected one of 'api', 'spi' or 'other'", (StringRef))
+      "expected one of 'api', 'spi', 'ipi', or 'other'", (StringRef))
 
 ERROR(error_unknown_require_explicit_availability, none,
       "unknown argument '%0', passed to -require-explicit-availability, "

include/swift/Basic/LangOptions.h

@@ -85,6 +85,10 @@ namespace swift {
     /// all decls in the module are considered to be SPI including public ones.
     SPI,
 
+    /// Internal Programming Interface that is not distributed and only usable
+    /// from within a project.
+    IPI,
+
     /// The library has some other undefined distribution.
     Other
   };

lib/Frontend/CompilerInvocation.cpp

@@ -704,17 +704,15 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       Opts.LibraryLevel = LibraryLevel::API;
     } else if (contents == "spi") {
       Opts.LibraryLevel = LibraryLevel::SPI;
+    } else if (contents == "ipi") {
+      Opts.LibraryLevel = LibraryLevel::IPI;
     } else {
       Opts.LibraryLevel = LibraryLevel::Other;
       if (contents != "other") {
         // Error on unknown library levels.
-        auto inFlight = Diags.diagnose(SourceLoc(),
-                                       diag::error_unknown_library_level,
-                                       contents);
-
-        // Only warn for "ipi" as we may use it in the future.
-        if (contents == "ipi")
-          inFlight.limitBehavior(DiagnosticBehavior::Warning);
+        Diags.diagnose(SourceLoc(),
+                       diag::error_unknown_library_level,
+                       contents);
       }
     }
   }

test/Sema/implementation-only-import-suggestion.swift

@@ -34,6 +34,9 @@
 // RUN: %target-swift-frontend -typecheck -sdk %t/sdk %t/PublicImports.swift \
 // RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ -module-cache-path %t \
 // RUN:   -library-level other -module-name MainLib
+// RUN: %target-swift-frontend -typecheck -sdk %t/sdk %t/PublicImports.swift \
+// RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ -module-cache-path %t \
+// RUN:   -library-level ipi -module-name MainLib
 //--- PublicImports.swift
 import PublicSwift
 import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'MainLib'}}
@@ -70,7 +73,7 @@ import LocalClang // expected-error{{private module 'LocalClang' is imported pub
 /// Test error message on an unknown library level name.
 // RUN: not %target-swift-frontend -typecheck %s -library-level ThatsNotALibraryLevel 2>&1 \
 // RUN:   | %FileCheck %s --check-prefix CHECK-ARG
-// CHECK-ARG: error: unknown library level 'ThatsNotALibraryLevel', expected one of 'api', 'spi' or 'other'
+// CHECK-ARG: error: unknown library level 'ThatsNotALibraryLevel', expected one of 'api', 'spi', 'ipi', or 'other'
 
 /// Expect no errors in swiftinterfaces.
 // RUN: %target-swift-typecheck-module-from-interface(%t/Client.private.swiftinterface) \