[AutoDiff] Emit SIL diff witnesses into original function's IRGenModule. (#28237)

The differentiation transform does not yet canonicalize differentiability
witnesses so that they always have JVP/VJP functions. TF-894 tracks this.

Until then, emit SIL differentiability witness into the IRGenModule for
the original function. The significance of this is unclear;
`IRGenerator::getGenModule` seems relevant only for multi-threaded
compilation.

Resolves TF-960: crash when `SILDifferentiabilityWitness::getVJP` is null.

Author: dan-zheng

lib/IRGen/GenDecl.cpp

@@ -1072,8 +1072,13 @@ void IRGenerator::emitGlobalTopLevel() {
     if (dw.isDeclaration())
       continue;
 
-    // Emit into same IRGenModule as the VJP.
-    CurrentIGMPtr IGM = getGenModule(dw.getVJP());
+    // Emit into same IRGenModule as the original function.
+    // NOTE(TF-894): Investigate whether `getGenModule(dw.getVJP())` is
+    // significant/desirable; `getGenModule` seems relevant for multi-threaded
+    // compilation. When the differentiation transform canonicalizes all
+    // differentiability witnesses to have JVP/VJP functions, we can assert
+    // that JVP/VJP functions exist and use `getGenModule(dw.getVJP())`.
+    CurrentIGMPtr IGM = getGenModule(dw.getOriginalFunction());
 
     IGM->emitSILDifferentiabilityWitness(&dw);
   }

test/AutoDiff/compiler_crashers_fixed/Inputs/tf960-irgen-diff-witness-no-derivatives-other-module.swift

@@ -0,0 +1,6 @@
+struct TF_960: Differentiable {
+  @differentiable
+  func callAsFunction(_ input: Float) -> Float {
+    return input
+  }
+}

test/AutoDiff/compiler_crashers_fixed/tf960-irgen-diff-witness-no-derivatives-cross-module.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend -c %S/Inputs/tf960-irgen-diff-witness-no-derivatives-other-module.swift %s -O -module-name main -num-threads 36
+// REQUIRES: asserts
+
+// TF-960: IRGen crash for uncanonicalized differentiability witnesses.
+// This issue will become obsolete after TF-894, when the differentiation
+// transform canonicalizes differentiability witnesses to have derivative
+// functions and assertions are added to IRGen.
+
+// Stack dump:
+// ...
+// 1.	Swift version 5.1.1-dev (Swift af915c09de)
+// 0  swiftc                   0x0000000109acda65 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
+// 1  swiftc                   0x0000000109acca68 llvm::sys::RunSignalHandlers() + 248
+// 2  swiftc                   0x0000000109ace058 SignalHandler(int) + 264
+// 3  libsystem_platform.dylib 0x00007fff728e4b5d _sigtramp + 29
+// 4  libsystem_platform.dylib 0x00007ff639560230 _sigtramp + 3334977264
+// 5  swiftc                   0x0000000105b316bb swift::irgen::IRGenerator::emitGlobalTopLevel() + 1307
+// 6  swiftc                   0x0000000105bf2572 swift::performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::LLVMContext&, llvm::ArrayRef<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > >, llvm::GlobalVariable**) + 1682