[silgen] Emit the location of the original function def if SILGen is erroring on a duplicate symbol definition.

This diagnostic is useful around silgen_name where it validates that we do not
have any weird collisions. Sadly, it just points where one of the conflicting
elements is... with this patch, we also emit an error on the other function if
we have a SILLocation.

Author: gottesmm

include/swift/AST/DiagnosticsSIL.def

@@ -46,6 +46,9 @@ ERROR(bridging_objcbridgeable_broken,none,
 ERROR(sil_function_redefinition,none,
       "multiple definitions of symbol '%0'",
       (StringRef))
+NOTE(sil_function_redefinition_note,none,
+      "other definition here",
+      ())
 
 ERROR(invalid_sil_builtin,none,
       "INTERNAL ERROR: invalid use of builtin: %0",

lib/SILGen/SILGen.cpp

@@ -813,6 +813,8 @@ void SILGenModule::emitFunctionDefinition(SILDeclRef constant, SILFunction *f) {
   if (!f->empty()) {
     diagnose(constant.getAsRegularLocation(), diag::sil_function_redefinition,
              f->getName());
+    if (f->hasLocation())
+      diagnose(f->getLocation(), diag::sil_function_redefinition_note);
     return;
   }
 

test/SILGen/double_silgen_name.swift

@@ -0,0 +1,7 @@
+// RUN: %target-swift-frontend -emit-silgen -verify %s
+
+@_silgen_name("first_func")
+func test() { } // expected-note {{other definition here}}
+
+@_silgen_name("first_func")
+func test2() { } // expected-error {{multiple definitions of symbol 'first_func'}}