Frontend: Implement optional parsing diagnostics for enabled language features.

Parsing for `-enable-upcoming-feature` and `-enable-experimental-feature` is
lenient by default because some projects need to be compatible with multiple
language versions and compiler toolchains simultaneously, and strict
diagnostics would be a nuisance. On the other hand, though, it would be useful
to get feedback from the compiler when you attempt to enable a feature that
doesn't exist. This change splits the difference by introducing new diagnostics
for potential feature enablement misconfigurations but leaves those diagnostics
ignored by default. Projects that wish to use them can specify `-Wwarning
StrictLanguageFeatures`.

include/swift/AST/DiagnosticGroups.def

@@ -26,6 +26,7 @@ GROUP(DeprecatedDeclaration, "DeprecatedDeclaration.md")
 GROUP(Unsafe, "Unsafe.md")
 GROUP(UnknownWarningGroup, "UnknownWarningGroup.md")
 GROUP(PreconcurrencyImport, "PreconcurrencyImport.md")
+GROUP(StrictLanguageFeatures, "StrictLanguageFeatures.md")
 
 #define UNDEFINE_DIAGNOSTIC_GROUPS_MACROS
 #include "swift/AST/DefineDiagnosticGroupsMacros.h"

include/swift/AST/DiagnosticsFrontend.def

@@ -37,8 +37,18 @@ ERROR(error_unsupported_target_arch, none,
       "unsupported target architecture: '%0'", (StringRef))
 
 WARNING(warning_upcoming_feature_on_by_default, none,
-      "upcoming feature '%0' is already enabled as of Swift version %1",
-      (StringRef, unsigned))
+        "upcoming feature '%0' is already enabled as of Swift version %1",
+        (StringRef, unsigned))
+
+GROUPED_WARNING(unrecognized_feature, StrictLanguageFeatures, DefaultIgnore,
+                "'%0' is not a recognized "
+                "%select{experimental|upcoming}1 feature",
+                (StringRef, bool))
+
+GROUPED_WARNING(feature_not_experimental, StrictLanguageFeatures, DefaultIgnore,
+                "'%0' is not an experimental feature, "
+                "use -%select{disable|enable}1-upcoming-feature instead",
+                (StringRef, bool))
 
 ERROR(error_unknown_library_level, none,
       "unknown library level '%0', "