[Serialization] Don't filter out unextended-module-map VFS in explicit builds.

In an explicit build LLDB needs to be able import the unmodified .pcms, so
having the exact same flags matters there, and there is no risk of a
recompilation failure, because nothing is recompiled.

rdar://136759808

Author: adrian-prantl

lib/Serialization/Serialization.cpp

@@ -1167,14 +1167,19 @@ void Serializer::writeHeader() {
         auto &Opts = Options.ExtraClangOptions;
         for (auto Arg = Opts.begin(), E = Opts.end(); Arg != E; ++Arg) {
           StringRef arg(*Arg);
-          if (arg.starts_with("-ivfsoverlay")) {
+          if (!Options.ExplicitModuleBuild && arg.starts_with("-ivfsoverlay")) {
             // FIXME: This is a hack and calls for a better design.
             //
             // Filter out any -ivfsoverlay options that include an
             // unextended-module-overlay.yaml overlay. By convention the Xcode
             // buildsystem uses these while *building* mixed Objective-C and
             // Swift frameworks; but they should never be used to *import* the
             // module defined in the framework.
+            //
+            // This is not done for explicit modules builds. In an explicit
+            // build LLDB needs to be able import the unmodified .pcms, so
+            // having the exact same flags matters there, and there is no risk
+            // of a recompilation failure, because nothing is recompiled.
             auto Next = std::next(Arg);
             if (Next != E &&
                 StringRef(*Next).ends_with("unextended-module-overlay.yaml")) {

test/Serialization/search-paths-relative.swift

@@ -6,6 +6,7 @@
 
 // Output paths differ in the new driver, so force SWIFT_USE_OLD_DRIVER for now.
 // RUN: cd %t/secret && env SWIFT_USE_OLD_DRIVER=1 %target-swiftc_driver -emit-module -o %t/has_xref.swiftmodule -I . -F ../Frameworks -parse-as-library %S/Inputs/has_xref.swift %S/../Inputs/empty.swift -Xfrontend -serialize-debugging-options -Xcc -ivfsoverlay -Xcc %S/../Inputs/unextended-module-overlay.yaml -Xcc -DDUMMY
+// RUN: cd %t/secret && env SWIFT_USE_OLD_DRIVER=1 %target-swiftc_driver -emit-module -o %t/explicit.swiftmodule -parse-stdlib -parse-as-library %S/../Inputs/empty.swift -Xfrontend -disable-implicit-swift-modules -Xfrontend -serialize-debugging-options -Xcc -ivfsoverlay -Xcc %S/../Inputs/unextended-module-overlay.yaml -Xcc -DDUMMY
 // RUN: %target-swift-frontend %s -typecheck -I %t
 
 // Ensure that in Swift 6 mode we do not read out search paths, thus are no longer able to
@@ -15,8 +16,10 @@
 
 // Check the actual serialized search paths.
 // RUN: llvm-bcanalyzer -dump %t/has_xref.swiftmodule > %t/has_xref.swiftmodule.txt
+// RUN: llvm-bcanalyzer -dump %t/explicit.swiftmodule > %t/explicit.swiftmodule.txt
 // RUN: %FileCheck %s < %t/has_xref.swiftmodule.txt
 // RUN: %FileCheck -check-prefix=NEGATIVE %s < %t/has_xref.swiftmodule.txt
+// RUN: %FileCheck -check-prefix=EXPLICIT %s < %t/explicit.swiftmodule.txt
 
 import has_xref
 
@@ -36,6 +39,10 @@ numeric(42)
 // NEGATIVE-NOT: '.'
 // NEGATIVE-NOT: '../Frameworks'
 // This should be filtered out.
-// NEGATIVE-NOT: -ivfsoverlay{{.*}}unextended-module-overlay.yaml
+// NEGATIVE-NOT: -ivfsoverlay
+// NEGATIVE-NOT: unextended-module-overlay.yaml
+// EXPLICIT: -ivfsoverlay
+// EXPLICIT: unextended-module-overlay.yaml
+// EXPLICIT: -DDUMMY
 
 // SWIFT6: error: missing required modules: 'has_alias', 'struct_with_operators'