Remove a hack to change linkage from public_external to shared.

The linkage change let the compiler generate code for public functions which are imported from the stdlib - and are also available in the swiftCore library.
This got worse since we use public linkage for @_versioned internal functions in the stdlib.

Getting rid of the linkage change reduces code size a lot: up to 40% for some projects.

I didn’t see any significant impact on benchmark performance.

Author: eeckstein

lib/SILOptimizer/Transforms/SILCleanup.cpp

@@ -47,12 +47,6 @@ static void cleanFunction(SILFunction &Fn) {
       }
     }
   }
-
-  // Rename functions with public_external linkage to prevent symbol conflict
-  // with stdlib.
-  if (Fn.isDefinition() && Fn.getLinkage() == SILLinkage::PublicExternal) {
-    Fn.setLinkage(SILLinkage::SharedExternal);
-  }
 }
 
 namespace {

test/IRGen/sil_linkage.sil

@@ -8,15 +8,15 @@ sil_stage canonical
 // CHECK: define{{( protected)?}} swiftcc void @hidden_fragile_function_test() {{.*}} {
 // CHECK: define linkonce_odr hidden swiftcc void @shared_fragile_function_test() {{.*}} {
 // CHECK: define{{( protected)?}} swiftcc void @private_fragile_function_test() {{.*}} {
-// CHECK: define linkonce_odr hidden swiftcc void @public_external_fragile_function_def_test() {{.*}} {
+// CHECK: define available_externally swiftcc void @public_external_fragile_function_def_test() {{.*}} {
 // CHECK: define{{( protected)?}} available_externally swiftcc void @hidden_external_fragile_function_def_test() {{.*}} {
 // CHECK: define linkonce_odr hidden swiftcc void @shared_external_fragile_function_def_test() {{.*}} {
 // CHECK: define{{( protected)?}} available_externally swiftcc void @private_external_fragile_function_def_test() {{.*}} {
 // CHECK: define{{( protected)?}} swiftcc void @public_resilient_function_test() {{.*}} {
 // CHECK: define hidden swiftcc void @hidden_resilient_function_test() {{.*}} {
 // CHECK: define linkonce_odr hidden swiftcc void @shared_resilient_function_test() {{.*}} {
 // CHECK: define internal swiftcc void @private_resilient_function_test() {{.*}}{
-// CHECK: define linkonce_odr hidden swiftcc void @public_external_resilient_function_def_test() {{.*}} {
+// CHECK: define available_externally swiftcc void @public_external_resilient_function_def_test() {{.*}} {
 // CHECK: define{{( protected)?}} available_externally hidden swiftcc void @hidden_external_resilient_function_def_test() {{.*}} {
 // CHECK: define linkonce_odr hidden swiftcc void @shared_external_resilient_function_def_test() {{.*}} {