Modules: Let -fmodules-decluse ignore headers that aren't in a module

Includes might always pull in arbitrary header or data files outside of
modules. Among others, this includes builtin includes, which do not have
a module (story) yet.

Also cleanup implementation of ModuleMap::findModuleForHeader() to be
non-recursive.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197034 91177308-0d34-0410-b5e6-96231b3b80d8