Allow running tests when stdlib is built with LTO by working around autolinking (swiftlang#34975)

Author: kubamracek

test/lit.cfg

@@ -945,6 +945,21 @@ if run_vendor == 'apple':
         config.resource_dir_opt = "-resource-dir %s" % new_resource_dir
         lit_config.note('Using freestanding resource dir: ' + new_resource_dir)
 
+    # Auto-linking does not work when stdlib is built with LTO, because linked
+    # libraries are discovered too late (after optimizations are applied). Let's
+    # explicitly link all libraries needed in tests.
+    #
+    # Note that this won't work on Linux (without LLD). The autolinking
+    # mechanism on Linux will fail due to needing -L arguments to the path even
+    # with this forced linking as the extracted commands will force the linker
+    # to look up the linking by name as well.
+    if "stdlib_lto" in config.available_features:
+        for library in ["swiftCore", "swiftStdlibUnittest",
+                        "swiftSwiftPrivateLibcExtras", "swiftSwiftPrivate",
+                        "swiftDarwin", "swiftSwiftPrivateThreadExtras",
+                        "swiftSwiftOnoneSupport"]:
+            swift_execution_tests_extra_flags += ' -Xlinker -l%s'% library
+
     xcrun_prefix = (
         "xcrun --toolchain %s --sdk %r" %
         (config.darwin_xcrun_toolchain, config.variant_sdk))

test/lit.site.cfg.in

@@ -81,6 +81,9 @@ if '@SWIFT_TOOLS_ENABLE_LTO@'.lower() in ['full', 'thin']:
 else:
     config.available_features.add('no_lto')
 
+if '@SWIFT_STDLIB_ENABLE_LTO@'.lower() in ['full', 'thin']:
+    config.available_features.add('stdlib_lto')
+
 if "@LLVM_ENABLE_ASSERTIONS@" == "TRUE":
     config.available_features.add('asserts')
 else: