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