[clang] Don't create import decls without -fmodules

When modules are disabled, there's no loaded module for these import
decls to point at. This results in crashes when there are modulemap
files but no -fmodules flag (this configuration is used for layering
check violations).

This patch makes sure import declarations are introduced only when
modules are enabled, which makes this case similar to textual headers
(no import decls are created for #include of textual headers from a
modulemap).

Differential Revision: https://reviews.llvm.org/D152274