[FrontendOptions] Add a frontend flag -enable-explicit-existential-types

to enable the 'any' keyword.

Author: hborla

include/swift/Basic/LangOptions.h

@@ -310,6 +310,10 @@ namespace swift {
     /// `func f() -> <T> T`.
     bool EnableExperimentalNamedOpaqueTypes = false;
 
+    /// Enable support for explicit existential types via the \c any
+    /// keyword.
+    bool EnableExplicitExistentialTypes = false;
+
     /// Enable experimental flow-sensitive concurrent captures.
     bool EnableExperimentalFlowSensitiveConcurrentCaptures = false;
 

include/swift/Option/FrontendOptions.td

@@ -472,6 +472,10 @@ def enable_experimental_structural_opaque_types :
   Flag<["-"], "enable-experimental-structural-opaque-types">,
   HelpText<"Enable experimental support for structural opaque result types">;
 
+def enable_explicit_existential_types :
+  Flag<["-"], "enable-explicit-existential-types">,
+  HelpText<"Enable experimental support for explicit existential types">;
+
 def enable_deserialization_recovery :
   Flag<["-"], "enable-deserialization-recovery">,
   HelpText<"Attempt to recover from missing xrefs (etc) in swiftmodules">;

lib/Frontend/CompilerInvocation.cpp

@@ -438,6 +438,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalNamedOpaqueTypes |=
       Args.hasArg(OPT_enable_experimental_named_opaque_types);
 
+  Opts.EnableExplicitExistentialTypes |=
+      Args.hasArg(OPT_enable_explicit_existential_types);
+
   Opts.EnableExperimentalDistributed |=
     Args.hasArg(OPT_enable_experimental_distributed);