Honor #sourceLocation filenames in serialized diagnostics

I'm not sure why we never handled this. Oops.

rdar://problem/43647151

Author: jrose-apple

lib/Frontend/SerializedDiagnosticConsumer.cpp

@@ -497,7 +497,7 @@ emitDiagnosticMessage(SourceManager &SM,
 
   StringRef filename = "";
   if (Loc.isValid())
-    filename = SM.getIdentifierForBuffer(SM.findBufferContainingLoc(Loc));
+    filename = SM.getDisplayNameForLoc(Loc);
 
   // Emit the RECORD_DIAG record.
   Record.clear();

test/Misc/serialized-diagnostics.swift

@@ -5,20 +5,23 @@
 // RUN: c-index-test -read-diagnostics %t.dia > %t.deserialized_diagnostics.txt 2>&1
 // RUN: %FileCheck --input-file=%t.deserialized_diagnostics.txt %s
 
-// Test swift_driver integrated frontend
-// RUN: %target-swift-frontend -typecheck -serialize-diagnostics-path %t.integrated_frontend.dia %s -verify
-// RUN: c-index-test -read-diagnostics %t.integrated_frontend.dia > %t.integrated_frontend.deserialized_diagnostics.txt 2>&1
-// RUN: %FileCheck --input-file=%t.integrated_frontend.deserialized_diagnostics.txt %s
-
 var x = 1 x = 2   // expected-error {{consecutive statements on a line must be separated by ';'}} {{10-10=;}}
-var z : Int       // expected-note {{previously declared here}}
+// CHECK: {{.*[/\\]}}serialized-diagnostics.swift:[[@LINE-1]]:10: error: consecutive statements on a line must be separated by ';' [] []
+// CHECK-NEXT: Number FIXITs = 1
+// CHECK-NEXT: FIXIT: ({{.*[/\\]}}serialized-diagnostics.swift:[[@LINE-3]]:10 - {{.*[/\\]}}serialized-diagnostics.swift:[[@LINE-3]]:10): ";"
+
+var z : Int       // expected-note 2 {{previously declared here}}
 var z : Int       // expected-error {{invalid redeclaration}}
+// CHECK-NEXT: {{.*[/\\]}}serialized-diagnostics.swift:[[@LINE-1]]:5: error: invalid redeclaration of 'z' [] []
+// CHECK-NEXT: Number FIXITs = 0
+// CHECK-NEXT: +-{{.*[/\\]}}serialized-diagnostics.swift:[[@LINE-4]]:5: note: 'z' previously declared here [] []
+// CHECK-NEXT: Number FIXITs = 0
 
-// CHECK: 13:10: error: consecutive statements on a line must be separated by ';' [] []
-// CHECK-NEXT: Number FIXITs = 1
-// CHECK-NEXT: FIXIT: ({{.*[/\\]}}serialized-diagnostics.swift:13:10 - {{.*[/\\]}}serialized-diagnostics.swift:13:10): ";"
-// CHECK-NEXT: 15:5: error: invalid redeclaration of 'z' [] []
+#sourceLocation(file: "fake-file.swuft", line: 4)
+var z : Int // Note: no expected-* here because it's "not in this file".
+// CHECK-NEXT: {{^}}fake-file.swuft:4:5: error: invalid redeclaration of 'z' [] []
 // CHECK-NEXT: Number FIXITs = 0
-// CHECK-NEXT: +-{{.*[/\\]}}serialized-diagnostics.swift:14:5: note: 'z' previously declared here [] []
+// CHECK-NEXT: +-{{.*[/\\]}}serialized-diagnostics.swift:{{[0-9]+}}:5: note: 'z' previously declared here [] []
 // CHECK-NEXT: Number FIXITs = 0
-// CHECK-NEXT: Number of diagnostics: 2
+
+// CHECK-NEXT: Number of diagnostics: 3