[Backtracing][Linux][Tests] Fix verify_all_overlays test.

The `_Backtracing` module has a number of private implementation only
imports that aren't used outside of the module and that don't require
any additional libraries (hence they aren't relevant to the outside
world).  `verify_all_overlays.py` needs to know about these when it
does its test, because it loadas the module as the main module, which
results in implementation only imports being required instead of
ignored.

rdar://110261712

Author: al45tair

validation-test/SIL/verify_all_overlays.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python3
 # RUN: ${python} %s %target-swiftmodule-name %platform-sdk-overlay-dir \
+# RUN:     %swift_src_root \
 # RUN:     %target-sil-opt -sdk %sdk -enable-sil-verify-all \
 # RUN:       -F %sdk/System/Library/PrivateFrameworks \
 # RUN:       -F "%xcode-extra-frameworks-dir"
@@ -14,9 +15,11 @@
 
 target_swiftmodule_name = sys.argv[1]
 sdk_overlay_dir = sys.argv[2]
-sil_opt_invocation = sys.argv[3:]
+source_dir = sys.argv[3]
+sil_opt_invocation = sys.argv[4:]
 
 for module_file in os.listdir(sdk_overlay_dir):
+    extra_args = []
     module_name, ext = os.path.splitext(module_file)
     if ext != ".swiftmodule":
         continue
@@ -33,6 +36,12 @@
     # TODO: fix the DifferentiationUnittest module.
     if module_name == "DifferentiationUnittest":
         continue
+    # Backtracing needs its own additional modules in the module path
+    if module_name == "_Backtracing":
+        extra_args = ["-I", os.path.join(source_dir, "stdlib",
+                                         "public", "Backtracing", "modules"),
+                      "-I", os.path.join(source_dir, "include")]
+
     print("# " + module_name)
 
     module_path = os.path.join(sdk_overlay_dir, module_file)
@@ -50,4 +59,5 @@
     # We are deliberately discarding the output here; we're just making sure
     # it can be generated.
     subprocess.check_output(sil_opt_invocation +
-                            [module_path, "-module-name", module_name])
+                            [module_path, "-module-name", module_name] +
+                            extra_args)