[lldb] Fix a regression in SBValue::GetObjectDescription() (llvm#117242)

The old behavior was to return a null string in the error case,when
refactoring the error handling I thought it would be a good idea to
print the error in the description, but that breaks clients that try to
print a description first and then do something else in the error case.
The API is not great but it's clear that in-band errors are also not a
good idea.

rdar://133956263
(cherry picked from commit 7553fb1)

 Conflicts:
	lldb/test/API/commands/expression/diagnostics/TestExprDiagnostics.py

(cherry picked from commit 091aa23)

Author: adrian-prantl

lldb/test/API/lang/swift/po/uninitialized/main.swift

@@ -15,7 +15,7 @@ class POClass {
 
 func main() {
   var object: POClass
-  object = POClass() //% self.assertTrue(self.frame().FindVariable('object').GetObjectDescription() == 'error: <uninitialized>', 'po correctly detects uninitialized instances')
+  object = POClass() //% self.assertEqual(self.frame().FindVariable('object').GetObjectDescription(), None, 'po correctly detects uninitialized instances'); self.expect("po object", substrs=["<uninitialized>"])
   print("yay I am done") //% self.assertTrue('POClass:' in self.frame().FindVariable('object').GetObjectDescription())
 }