Add a test that serialized diagnostics respect VFS filenames too

Author: jrose-apple

test/Frontend/vfs.swift

@@ -3,11 +3,13 @@
 // RUN: sed -e "s:INPUT_DIR:%S/Inputs:g" -e "s:OUT_DIR:%t:g" %S/Inputs/vfs/secondary-vfsoverlay.yaml > %t/secondary-overlay.yaml
 // RUN: sed -e "s:INPUT_DIR:%S/Inputs:g" -e "s:OUT_DIR:%t:g" %S/Inputs/vfs/tertiary-vfsoverlay.yaml > %t/tertiary-overlay.yaml
 
-// RUN: not %target-swift-frontend -vfsoverlay %t/overlay.yaml -typecheck %s %t/mapped-file.swift 2>&1 | %FileCheck -check-prefix=BASIC_MAPPING_ERROR %s
+// RUN: not %target-swift-frontend -vfsoverlay %t/overlay.yaml -typecheck %s %t/mapped-file.swift -serialize-diagnostics-path %t/basic.dia 2>&1 | %FileCheck -check-prefix=BASIC_MAPPING_ERROR %s
+// RUN: c-index-test -read-diagnostics %t/basic.dia 2>&1 | %FileCheck -check-prefix=BASIC_MAPPING_ERROR %s
 
 // BASIC_MAPPING_ERROR: {{.*}}/mapped-file.swift:2:17: error:
 
-// RUN: not %target-swift-frontend -vfsoverlay %t/overlay.yaml -vfsoverlay %t/secondary-overlay.yaml -vfsoverlay %t/tertiary-overlay.yaml -typecheck %s %t/triple-mapped-swift-file.swift 2>&1 | %FileCheck -check-prefix=COMPLEX_MAPPING_ERROR %s
+// RUN: not %target-swift-frontend -vfsoverlay %t/overlay.yaml -vfsoverlay %t/secondary-overlay.yaml -vfsoverlay %t/tertiary-overlay.yaml -typecheck %s %t/triple-mapped-swift-file.swift -serialize-diagnostics-path %t/complex.dia 2>&1 | %FileCheck -check-prefix=COMPLEX_MAPPING_ERROR %s
+// RUN: c-index-test -read-diagnostics %t/complex.dia 2>&1 | %FileCheck -check-prefix=COMPLEX_MAPPING_ERROR %s
 
 // COMPLEX_MAPPING_ERROR: {{.*}}/triple-mapped-swift-file.swift:2:17: error: