Simplify FindExternalVisibleDeclsByName by making it return a bool indicating
if it found any decls, rather than returning a list of found decls. This
removes a returning-ArrayRef-to-deleted-storage bug from
MultiplexExternalSemaSource (in code not exercised by any of the clang
binaries), reduces the work required in the found-no-decls case with PCH, and
importantly removes the need for DeclContext::lookup to be reentrant.

No functionality change intended!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174576 91177308-0d34-0410-b5e6-96231b3b80d8
Fix handling of module imports adding names to a DeclContext after qualified
name lookup has been performed in that context (this probably only happens in
C++).

1) Whenever we add names to a context, set a flag on it, and if we perform
lookup and discover that the context has had a lookup table built but has the
flag set, update all entries in the lookup table with additional names from
the external source.

2) When marking a DeclContext as having external visible decls, mark the
context in which lookup is performed, not the one we are adding. These won't
be the same if we're adding another copy of a pre-existing namespace.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174577 91177308-0d34-0410-b5e6-96231b3b80d8