SIL: Relax some verifier checks when merging partial modules

When merging partial SIL modules we only link in function bodies defined
in the current module, so we might encounter functions with shared
linkage and no body.

Since pulling in these functions from other modules is a waste of time,
run the verifier in "single function" mode in this case.

Note that when the module is ultimately used and one of these functions
is deserialized, we should link in all downstream functions with shared
linkage, and failure to do so will be caught by the SIL verifier then.

Fixes <rdar://problem/34469704>.

Author: slavapestov

lib/SIL/SILVerifier.cpp

@@ -4516,13 +4516,20 @@ void SILModule::verify() const {
   // Uniquing set to catch symbol name collisions.
   llvm::StringSet<> symbolNames;
 
+  // When merging partial modules, we only link functions from the current
+  // module, without enabling "LinkAll" mode or running the SILLinker pass;
+  // in this case, we need to relax some of the checks.
+  bool SingleFunction = false;
+  if (getOptions().MergePartialModules)
+    SingleFunction = true;
+
   // Check all functions.
   for (const SILFunction &f : *this) {
     if (!symbolNames.insert(f.getName()).second) {
       llvm::errs() << "Symbol redefined: " << f.getName() << "!\n";
       assert(false && "triggering standard assertion failure routine");
     }
-    f.verify(/*SingleFunction=*/false);
+    f.verify(SingleFunction);
   }
 
   // Check all globals.

test/Frontend/sil-merge-partial-modules-stdlib.swift

@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -emit-module -primary-file %s -module-name test -o %t/partial.swiftmodule -O
+
+// RUN: %target-swift-frontend -emit-module %t/partial.swiftmodule -module-name test -sil-merge-partial-modules -disable-diagnostic-passes -disable-sil-perf-optzns -o %t/test.swiftmodule
+
+public func makeMirror(object x: Any) -> Mirror {
+  return Mirror(reflecting: x)
+}