[flang] Restore accidentally deleted test. NFC.

Somehow this file got deleted in 5d43f61, while resolving a merge
conflict.  I have no idea how, and my abundant terminal scrollback for
this branch offers no explanation whatsoever.  It's quite disturbing.

rdar://166776360

Author: ahmedbougacha

flang/test/Semantics/modfile75.F90

@@ -0,0 +1,18 @@
+!RUN: rm -rf %t && mkdir -p %t
+!RUN: %flang -c -fhermetic-module-files -DWHICH=1 -J%t %s && %flang -c -fhermetic-module-files -DWHICH=2 -J%t %s && %flang_fc1 -fdebug-unparse -J%t %s | FileCheck %s
+
+#if WHICH == 1
+module modfile75a
+  use iso_c_binding
+end
+#elif WHICH == 2
+module modfile75b
+  use modfile75a
+end
+#else
+program test
+  use modfile75b
+!CHECK: INTEGER(KIND=4_4) n
+  integer(c_int) n
+end
+#endif