[Frontend] Add compatibility libraries to -print-target-info.

The driver and any other client that attempts to properly link Swift
code need to know which compatibility libraries should be linked on a
per-target basis. Vend that information as part of -print-target-info.

lib/FrontendTool/FrontendTool.cpp

@@ -1948,8 +1948,36 @@ createJSONFixItDiagnosticConsumerIfNeeded(
   });
 }
 
+/// Print information about a
+static void printCompatibilityLibrary(
+    llvm::VersionTuple runtimeVersion, llvm::VersionTuple maxVersion,
+    StringRef filter, StringRef libraryName, bool &printedAny,
+    llvm::raw_ostream &out) {
+  if (runtimeVersion > maxVersion)
+    return;
+
+  if (printedAny) {
+    out << ",";
+  }
+
+  out << "\n";
+  out << "      {\n";
+
+  out << "        \"libraryName\": \"";
+  out.write_escaped(libraryName);
+  out << "\",\n";
+
+  out << "        \"filter\": \"";
+  out.write_escaped(filter);
+  out << "\"\n";
+  out << "      }";
+
+  printedAny = true;
+}
+
 /// Print information about the target triple in JSON.
 static void printTripleInfo(const llvm::Triple &triple,
+                            llvm::Optional<llvm::VersionTuple> runtimeVersion,
                             llvm::raw_ostream &out) {
   out << "{\n";
 
@@ -1965,11 +1993,26 @@ static void printTripleInfo(const llvm::Triple &triple,
   out.write_escaped(getTargetSpecificModuleTriple(triple).getTriple());
   out << "\",\n";
 
-  if (auto runtimeVersion = getSwiftRuntimeCompatibilityVersionForTarget(
-          triple)) {
+  if (runtimeVersion) {
     out << "    \"swiftRuntimeCompatibilityVersion\": \"";
     out.write_escaped(runtimeVersion->getAsString());
     out << "\",\n";
+
+    // Compatibility libraries that need to be linked.
+    out << "    \"compatibilityLibraries\": [";
+    bool printedAnyCompatibilityLibrary = false;
+    #define BACK_DEPLOYMENT_LIB(Version, Filter, LibraryName)           \
+      printCompatibilityLibrary(                                        \
+        *runtimeVersion, llvm::VersionTuple Version, #Filter, LibraryName, \
+        printedAnyCompatibilityLibrary, out);
+    #include "swift/Frontend/BackDeploymentLibs.def"
+
+    if (printedAnyCompatibilityLibrary) {
+      out << "\n   ";
+    }
+    out << " ],\n";
+  } else {
+    out << "    \"compatibilityLibraries\": [ ],\n";
   }
 
   out << "    \"librariesRequireRPath\": "
@@ -1992,14 +2035,16 @@ static void printTargetInfo(const CompilerInvocation &invocation,
   out << "\",\n";
 
   // Target triple and target variant triple.
+  auto runtimeVersion =
+    invocation.getIRGenOptions().AutolinkRuntimeCompatibilityLibraryVersion;
   auto &langOpts = invocation.getLangOptions();
   out << "  \"target\": ";
-  printTripleInfo(langOpts.Target, out);
+  printTripleInfo(langOpts.Target, runtimeVersion, out);
   out << ",\n";
 
   if (auto &variant = langOpts.TargetVariant) {
     out << "  \"targetVariant\": ";
-    printTripleInfo(*variant, out);
+    printTripleInfo(*variant, runtimeVersion, out);
     out << ",\n";
   }