completes the support in the LLDB expression parser
for incomplete types.  Clang now imports types
lazily, and we complete those types as necessary.

Changes include:

- ClangASTSource now supports three APIs which it
  passes to ClangExpressionDeclMap.  CompleteType
  completes a TagDecl or an ObjCInterfaceDecl when
  needed; FindExternalVisibleDecls finds named
  entities that are visible in the expression's
  scope; and FindExternalLexicalDecls performs a
  (potentially restricted) search for entities
  inside a lexical scope like a namespace.  These
  changes mean that entities in namespaces should
  work normally.

- The SymbolFileDWARF code for searching a context
  for a specific name is now more general, and can
  search arbitrary contexts.

- We are continuing to adapt our calls into LLVM
  from interfaces that take start and end iterators
  when accepting multiple items to interfaces that
  use ArrayRef.

- I have cleaned up some code, especially our use
  of namespaces.

This change is neutral for our testsuite and greatly
improves correctness for large programs (like Clang)
with complicated type systems.  It should also lay
the groundwork for improving the expression parser's
performance as we are lazier and lazier about
providing type information.

llvm-svn: 136555