Use LLVM version string

[sas]

Summary

macOS builds of LLDB use the bundle version from
tools/driver/lldb-Info.plist. That file hasn't been updated since the 4.0
release so the version we print provides no value. I also think that even if it
were up to date, that number has no meaning and displaying the version from the
LLVM tree is more valuable.

I know that Apple folks have some form of override for the clang version to
match the Xcode version, so it'd make sense for them to do the same for LLDB.

lldb/trunk/source/lldb.cpp

 #define QUOTE(str) #str
 #define EXPAND_AND_QUOTE(str) QUOTE(str)
 
 const char *lldb_private::GetVersion() {
   // On platforms other than Darwin, report a version number in the same style
   // as the clang tool.
   static std::string g_version_str;
   if (g_version_str.empty()) {
-
-#ifdef LLDB_VERSION_STRING
-    g_version_str += EXPAND_AND_QUOTE(LLDB_VERSION_STRING);
-#else
     g_version_str += "lldb version ";
     g_version_str += CLANG_VERSION_STRING;
-#endif
+
     const char *lldb_repo = GetLLDBRepository();
     const char *lldb_rev = GetLLDBRevision();
     if (lldb_repo || lldb_rev) {
       g_version_str += " (";
       if (lldb_repo)
         g_version_str += lldb_repo;
       if (lldb_rev) {
         g_version_str += " revision ";