Make _unavailableFromAsync user inaccessible

Just making this user inaccessible until I go through the full proposal
pipeline.

Author: etcwilde

include/swift/AST/Attr.def

@@ -700,7 +700,7 @@ CONTEXTUAL_SIMPLE_DECL_ATTR(_const, CompileTimeConst,
   126)
 
 SIMPLE_DECL_ATTR(_unavailableFromAsync, UnavailableFromAsync,
-  OnFunc | OnConstructor |
+  OnFunc | OnConstructor | UserInaccessible |
   ABIStableToAdd | ABIStableToRemove |
   APIBreakingToAdd | APIStableToRemove,
   127)

test/IDE/complete_decl_attribute.swift

@@ -72,7 +72,6 @@ struct MyStruct {}
 // KEYWORD2-NEXT:             Keyword/None:                       transpose[#Func Attribute#]; name=transpose
 // KEYWORD2-NEXT:             Keyword/None:                       noDerivative[#Func Attribute#]; name=noDerivative
 // KEYWORD2-NEXT:             Keyword/None:                       Sendable[#Func Attribute#]; name=Sendable
-// KEYWORD2-NEXT:             Keyword/None:                       _unavailableFromAsync[#Func Attribute#]; name=_unavailableFromAsync
 // KEYWORD2-NOT:              Keyword
 // KEYWORD2:                  Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct
 // KEYWORD2:                  End completions
@@ -156,7 +155,6 @@ struct _S {
 // ON_INIT-DAG: Keyword/None:                       inlinable[#Constructor Attribute#]; name=inlinable
 // ON_INIT-DAG: Keyword/None:                       usableFromInline[#Constructor Attribute#]; name=usableFromInline
 // ON_INIT-DAG: Keyword/None:                       discardableResult[#Constructor Attribute#]; name=discardableResult
-// ON_INIT-DAG: Keyword/None:                       _unavailableFromAsync[#Constructor Attribute#]; name=_unavailableFromAsync
 // ON_INIT: End completions
 
   @#^ON_PROPERTY^# var foo
@@ -198,7 +196,6 @@ struct _S {
 // ON_METHOD-DAG: Keyword/None:                       transpose[#Func Attribute#]; name=transpose
 // ON_METHOD-DAG: Keyword/None:                       Sendable[#Func Attribute#]; name=Sendable
 // ON_METHOD-DAG: Keyword/None:                       noDerivative[#Func Attribute#]; name=noDerivative
-// ON_METHOD-DAG: Keyword/None:                       _unavailableFromAsync[#Func Attribute#]; name=_unavailableFromAsync
 // ON_METHOD-NOT: Keyword
 // ON_METHOD: Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct
 // ON_METHOD: End completions
@@ -260,7 +257,6 @@ struct _S {
 // ON_MEMBER_LAST-DAG: Keyword/None:                       transpose[#Declaration Attribute#]; name=transpose
 // ON_MEMBER_LAST-DAG: Keyword/None:                       noDerivative[#Declaration Attribute#]; name=noDerivative
 // ON_MEMBER_LAST-DAG: Keyword/None:                       Sendable[#Declaration Attribute#]; name=Sendable
-// ON_MEMBER_LAST-DAG: Keyword/None:                       _unavailableFromAsync[#Declaration Attribute#]; name=_unavailableFromAsync
 // ON_MEMBER_LAST-NOT: Keyword
 // ON_MEMBER_LAST: Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct
 // ON_MEMBER_LAST-NOT: Decl[PrecedenceGroup]
@@ -311,7 +307,6 @@ func dummy2() {}
 // KEYWORD_LAST-DAG: Keyword/None:                       transpose[#Declaration Attribute#]; name=transpose
 // KEYWORD_LAST-DAG: Keyword/None:                       noDerivative[#Declaration Attribute#]; name=noDerivative
 // KEYWORD_LAST-DAG: Keyword/None:                       Sendable[#Declaration Attribute#]; name=Sendable
-// KEYWORD_LAST-DAG: Keyword/None:                       _unavailableFromAsync[#Declaration Attribute#]; name=_unavailableFromAsync
 // KEYWORD_LAST-NOT: Keyword
 // KEYWORD_LAST: Decl[Struct]/CurrModule:            MyStruct[#MyStruct#]; name=MyStruct
 // KEYWORD_LAST:                  End completions