This patch (thanks to Doug Gregor) fixes a

problem where Clang was setting the
hasExternalVisibleDecls() bit for all
DeclContexts it imported.  This caused Clang
to make unnecessary calls to
findExternalVisibleDecls() when an external
AST source was installed.

In fact, Clang sometimes interpreted a failure
by one of these spurious calls to find a
Decl as meaning the Decl didn't exist, even
though findExternalLexicalDecls() did locate
that decl.  This produced amusing errors of
the form:

-
error: no member named 'b' in 'A'; did you
mean 'b'?
-

Now, if hasExternalVisibleDecls() or
hasExternalLexicalDecls() should be set, the
external AST source must do so itself.

llvm-svn: 135824