Add PrettyStackTraceSwiftVersion, and use it in the swiftc executable (#24235)

As pointed out on a recent JIRA, crash traces don't mention what
version of Swift you were running. Usually that can be gleaned from
the path, but not always.

lib/Basic/PrettyStackTrace.cpp

@@ -17,6 +17,7 @@
 
 #include "swift/Basic/PrettyStackTrace.h"
 #include "swift/Basic/QuotedString.h"
+#include "swift/Basic/Version.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/raw_ostream.h"
 
@@ -33,3 +34,7 @@ void PrettyStackTraceFileContents::print(llvm::raw_ostream &out) const {
     out << '\n';
   out << "---\n";
 }
+
+void PrettyStackTraceSwiftVersion::print(llvm::raw_ostream &out) const {
+  out << version::getSwiftFullVersion() << '\n';
+}