[Test] Add linker_overridden feature.

When testing different linkers, it's sometimes useful to run the tests
with `SWIFT_DRIVER_TEST_OPTIONS=" -use-ld=<linker>"`.  If we do this,
it will break a handful of tests because they expect the compiler driver
to choose an appropriate linker automatically.

To avoid having these fail, detect when someone has done this, and
set a new feature, `linker_overridden`, then mark the tests in question
with `UNSUPPORTED: linker_overridden`.

rdar://123504095

Author: al45tair

test/Driver/link-time-opt.swift

@@ -1,3 +1,5 @@
+// UNSUPPORTED: linker_overridden
+
 // RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -lto=llvm-thin -target x86_64-unknown-linux-gnu    | %FileCheck %s --check-prefix=CHECK-SIMPLE-THIN --check-prefix=CHECK-SIMPLE-THIN-linux-gnu
 // RUN: %swiftc_driver -driver-print-jobs %S/../Inputs/empty.swift -lto=llvm-thin -target x86_64-unknown-windows-msvc | %FileCheck %s --check-prefix=CHECK-SIMPLE-THIN --check-prefix=CHECK-SIMPLE-THIN-windows-msvc
 

test/Driver/static-archive.swift

@@ -1,3 +1,5 @@
+// UNSUPPORTED: linker_overridden
+
 // RUN: %swiftc_driver -driver-print-jobs -target x86_64-apple-macosx10.9 -emit-library %s -module-name ARCHIVER -static 2>&1 | %FileCheck -check-prefix CHECK-MACOS %s
 
 // CHECK-MACOS: swift

test/Interpreter/llvm_link_time_opt.swift

@@ -10,6 +10,8 @@
 // loaded too late").
 // REQUIRES: no_asan
 
+// UNSUPPORTED: linker_overridden
+
 // RUN: %empty-directory(%t)
 // RUN: %use_just_built_liblto %target-swiftc_driver -emit-library -static -lto=llvm-full %lto_flags -emit-module %S/Inputs/lto/module1.swift -working-directory %t
 // RUN: %use_just_built_liblto %target-swiftc_driver -lto=llvm-full %lto_flags %s -I%t -L%t -lmodule1 -module-name main -o %t/main

test/embedded/once.swift

@@ -6,6 +6,7 @@
 // REQUIRES: executable_test
 // REQUIRES: optimized_stdlib
 // REQUIRES: OS=macosx || OS=linux-gnu
+// UNSUPPORTED: linker_overridden
 
 // For LTO, the linker dlopen()'s the libLTO library, which is a scenario that
 // ASan cannot work in ("Interceptors are not working, AddressSanitizer is

test/lit.cfg

@@ -547,6 +547,11 @@ config.swift_driver_test_options += os.environ.get('SWIFT_DRIVER_TEST_OPTIONS',
 config.swift_ide_test_test_options += os.environ.get('SWIFT_IDE_TEST_TEST_OPTIONS', '')
 config.sil_test_options = os.environ.get('SIL_TEST_OPTIONS', '')
 
+# Check if we overrode the linker; if we do, set a feature to say that, which
+# lets us disable tests that rely on the driver choosing the linker.
+if re.search(r'(?:^|[ \t])-use-ld=[^ \t]*', config.swift_driver_test_options):
+    config.available_features.add('linker_overridden')
+
 config.clang_module_cache_path = make_path(config.swift_test_results_dir, "clang-module-cache")
 shutil.rmtree(config.clang_module_cache_path, ignore_errors=True)
 mcp_opt = "-module-cache-path %s" % shell_quote(config.clang_module_cache_path)