[Frontend] Add compiler version information to -print-target-info output.

Clients that use -print-target-info can avoid an extra frontend
invocation by using this information.

lib/FrontendTool/FrontendTool.cpp

@@ -1985,6 +1985,12 @@ static void printTargetInfo(const CompilerInvocation &invocation,
                             llvm::raw_ostream &out) {
   out << "{\n";
 
+  // Compiler version, as produced by --version.
+  out << "  \"compilerVersion\": \"";
+  out.write_escaped(version::getSwiftFullVersion(
+                                                 version::Version::getCurrentLanguageVersion()));
+  out << "\",\n";
+
   // Target triple and target variant triple.
   auto &langOpts = invocation.getLangOptions();
   out << "  \"target\": ";

test/Driver/print_target_info.swift

@@ -9,6 +9,8 @@
 
 // RUN: %swift_driver -print-target-info -target x86_64-apple-ios12.0 | %FileCheck -check-prefix CHECK-IOS-SIM %s
 
+// CHECK-IOS:   "compilerVersion": "Swift version
+
 // CHECK-IOS:   "target": {
 // CHECK-IOS:     "triple": "arm64-apple-ios12.0",
 // CHECK-IOS:     "unversionedTriple": "arm64-apple-ios",
@@ -28,6 +30,8 @@
 // CHECK-IOS:   }
 
 
+// CHECK-LINUX:   "compilerVersion": "Swift version
+
 // CHECK-LINUX:   "target": {
 // CHECK-LINUX:     "triple": "x86_64-unknown-linux",
 // CHECK-LINUX:     "moduleTriple": "x86_64-unknown-linux",