SIL: Set witness table linkage when parsing a witness table that already exists

I discovered this while testing the new commit, since it results in fewer
witness tables getting pre-declared.

There was a SIL test that relied on the old behavior; I'm going to explicitly
declare those witness tables as public_external to preserve behavior.

Author: slavapestov

lib/ParseSIL/ParseSIL.cpp

@@ -6184,6 +6184,8 @@ bool SILParserTUState::parseSILWitnessTable(Parser &P) {
 
   if (!wt)
     wt = SILWitnessTable::create(M, *Linkage, theConformance);
+  else
+    wt->setLinkage(*Linkage);
   wt->convertToDefinition(witnessEntries, conditionalConformances,
                           isSerialized);
   BodyScope.reset();

test/IRGen/existential_metatypes.sil

@@ -109,10 +109,10 @@ bb3:
   return %9 : $()
 }
 
-sil_witness_table Int: Kindable module existential_metatypes {
+sil_witness_table public_external Int: Kindable module existential_metatypes {
   method #Kindable.kind!getter.1: @int_kind_getter_witness
 }
 
-sil_witness_table Float: Kindable module existential_metatypes {
+sil_witness_table public_external Float: Kindable module existential_metatypes {
   method #Kindable.kind!getter.1: @float_kind_getter_witness
 }

test/SIL/Parser/witness_table_redeclare.sil

@@ -0,0 +1,12 @@
+// RUN: %target-sil-opt %s -module-name=witness_table_redeclare | %target-sil-opt -module-name=witness_table_redeclare | %FileCheck %s
+
+protocol P {}
+
+struct S : P {}
+
+sil_witness_table S: P module witness_table_redeclare
+
+sil_witness_table S: P module witness_table_redeclare {}
+
+// CHECK-LABEL: sil_witness_table S: P module witness_table_redeclare {
+// CHECK-NEXT: }