expression parser.  You can use a persistent
type like this:

(lldb) expr struct $foo { int a; int b; };
(lldb) struct $foo i; i.a = 2; i.b = 3; i
($foo) $0 = {
  (int) a = 2
  (int) b = 3
}

typedefs work similarly.

This patch affects the following files:

test/expression_command/persistent_types/*
  A test case for persistent types,
  in particular structs and typedefs.

ClangForward.h
  Added TypeDecl, needed to declare some
  functions in ASTResultSynthesizer.h

ClangPersistentVariables.[h,cpp]
  Added a list of persistent types to the
  persistent variable store.

ASTResultSynthesizer.[h,cpp]
  Made the AST result synthesizer iterate
  across TypeDecls in the expression, and
  record any persistent types found.  Also
  made a minor documentation fix.

ClangUserExpression.[h,cpp]
  Extended the user expression class to
  keep the state needed to report the
  persistent variable store for the target
  to the AST result synthesizers. 

  Also introduced a new error code for
  expressions that executed normally but
  did not return a result.

CommandObjectExpression.cpp
  Improved output for expressions (like 
  declarations of new persistent types) that
  don't return a result.  This is no longer
  treated as an error.

llvm-svn: 138383