SIL: Revert public linkage for @_silgen_name forward declarations.

It turns out that the stdlib build depends on `internal` functions with
`@_silgen_name` getting hidden linkage in some configurations. Instead of
messing with the linkage computation, just fix the `stdlib/Error.swift` test by
making `setWillThrowHandler` `public` to give it the right linkage.

Resolves rdar://141590619.

Author: tshortli

lib/SIL/IR/SILDeclRef.cpp

@@ -651,14 +651,6 @@ SILLinkage SILDeclRef::getDefinitionLinkage() const {
       effectiveAccess = std::max(effectiveAccess, AccessLevel::Internal);
   }
 
-  // Declarations with a @_silgen_name attribute and no body may be forward
-  // declarations of functions defined in another module. Therefore they must
-  // always have external (public) linkage, regardless of declared access level.
-  if (auto afd = getAbstractFunctionDecl()) {
-    if (!afd->hasBody() && afd->getAttrs().hasAttribute<SILGenNameAttr>())
-      effectiveAccess = AccessLevel::Public;
-  }
-
   switch (effectiveAccess) {
   case AccessLevel::Private:
   case AccessLevel::FilePrivate:

test/IRGen/silgen_name_linkage.swift

@@ -229,7 +229,7 @@ func throwJazzHandsTyped() throws(SillyError) {
 // not available, so pass it through an UnsafeRawPointer.
 @available(SwiftStdlib 5.8, *)
 @_silgen_name("_swift_setWillThrowHandler")
-func setWillThrowHandler(
+public func setWillThrowHandler(
     _ handler: (@convention(c) (UnsafeRawPointer) -> Void)?
 )