I added a new function to DeclContext called

addDeclInternal().  This function suppresses any
calls to FindExternalVisibleDeclsByName() while
a Decl is added to a DeclContext.  This behavior
is required for the ASTImporter, because in the
case of the LLDB client the ASTImporter would be
called recursively to import the visible decls,
which leads to assertions because the recursive
call is seeing partially-formed types.

I also modified the ASTImporter to use
addDeclInternal() in all places where it would
otherwise use addDecl().  This fix should not
affect the rest of Clang, passes Clang's
testsuite, and fixes several serious LLDB bugs.

llvm-svn: 142634