Add regression test for rust-lang#85955.

Author: michaelwoerister

src/test/ui/include-macros/auxiliary/same-file-in-two-crates-aux.rs

@@ -0,0 +1,4 @@
+#[inline]
+pub fn some_function() -> u32 {
+    1
+}

src/test/ui/include-macros/same-file-in-two-crates.rs

@@ -0,0 +1,21 @@
+// This test makes sure that the compiler can handle the same source file to be
+// part of the local crate *and* an upstream crate. This can happen, for example,
+// when there is some auto-generated code that is part of both a library and an
+// accompanying integration test.
+//
+// The test uses include!() to include a source file that is also part of
+// an upstream crate.
+//
+// This is a regression test for https://github.com/rust-lang/rust/issues/85955.
+
+// check-pass
+// compile-flags: --crate-type=rlib
+// aux-build:same-file-in-two-crates-aux.rs
+extern crate same_file_in_two_crates_aux;
+
+pub fn foo() -> u32 {
+    same_file_in_two_crates_aux::some_function() +
+    some_function()
+}
+
+include!("./auxiliary/same-file-in-two-crates-aux.rs");