[cxx-interop] Add llvm-lit feature 'std_span'

This adds a check in lit.local.cfg for whether the current C++ stdlib
contains the 'span' header. If so, the llvm-lit feature 'std_span' is
set. Also adds 'REQUIRES: std_span' to interop tests that include
'span'. This means we no longer have to choose between blanket disabling
`std::span` tests on all Linux distributions, or listing every Linux
distro with a libstdc++ version without `std::span` support as
unsupported.

I couldn't find any existing path to the MSVC stdlib in lit, and we
don't test versions of MSVC old enought to not support std::span
anyways, so 'std_span' is unconditionally enabled when targeting
windows-msvc.

rdar://161999160
rdar://161999174
rdar://162106580
rdar://162106619
rdar://162106643
rdar://162106653
rdar://162106722
rdar://162106747

Author: hnrklssn

test/Interop/Cxx/class/access/swiftify-private-fileid.swift

@@ -4,6 +4,7 @@
 // RUN: %target-swift-frontend -emit-ir -I %swift_src_root/lib/ClangImporter/SwiftBridging -plugin-path %swift-plugin-dir %t/blessed.swift -module-name main -I %t/Inputs -o %t/out -Xcc -std=c++20  -cxx-interoperability-mode=default -enable-experimental-feature SafeInteropWrappers -verify
 
 // REQUIRES: swift_feature_SafeInteropWrappers
+// REQUIRES: std_span
 
 //--- Inputs/swiftify-non-public.h
 #pragma once

test/Interop/Cxx/class/safe-interop-mode.swift

@@ -5,6 +5,7 @@
 
 // REQUIRES: objc_interop
 // REQUIRES: swift_feature_LifetimeDependence
+// REQUIRES: std_span
 
 //--- Inputs/module.modulemap
 module Test {

test/Interop/Cxx/stdlib/std-span-interface.swift

@@ -7,7 +7,7 @@
 
 // REQUIRES: swift_feature_SafeInteropWrappers
 // REQUIRES: swift_feature_Lifetimes
-// REQUIRES: rdar161999174
+// REQUIRES: std_span
 
 #if !BRIDGING_HEADER
 import StdSpan

test/Interop/Cxx/stdlib/std-span-transformed-execution.swift

@@ -7,6 +7,7 @@
 // REQUIRES: swift_feature_LifetimeDependence
 
 // REQUIRES: executable_test
+// REQUIRES: std_span
 
 #if !BRIDGING_HEADER
 import StdSpan

test/Interop/Cxx/stdlib/use-std-span-typechecker.swift

@@ -1,4 +1,5 @@
 // RUN: %target-typecheck-verify-swift -I %S/Inputs -enable-experimental-cxx-interop -Xcc -std=c++20 2>&1
+// REQUIRES: std_span
 
 import StdSpan
 

test/Interop/Cxx/stdlib/use-std-span.swift

@@ -5,6 +5,7 @@
 // UNSUPPORTED: OS=windows-msvc
 
 // REQUIRES: executable_test
+// REQUIRES: std_span
 
 import StdlibUnittest
 #if !BRIDGING_HEADER

test/Interop/Cxx/swiftify-import/import-as-instance-method.swift

@@ -1,4 +1,5 @@
 // REQUIRES: swift_feature_SafeInteropWrappers
+// REQUIRES: std_span
 
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t

test/Interop/Cxx/swiftify-import/span-in-ctor.swift

@@ -1,4 +1,5 @@
 // REQUIRES: swift_feature_SafeInteropWrappers
+// REQUIRES: std_span
 
 // RUN: rm -rf %t
 // RUN: split-file %s %t

test/Interop/CxxToSwiftToCxx/span/span-execution.cpp

@@ -10,6 +10,7 @@
 // RUN: %target-run %t/swift-cxx-execution | %FileCheck %s
 
 // REQUIRES: executable_test
+// REQUIRES: std_span
 
 //--- header.h
 #include <cstdint>

test/Interop/lit.local.cfg

@@ -44,9 +44,22 @@ else:
     config.substitutions.insert(0, ('%target-abi', 'SYSV'))
     config.substitutions.insert(0, ('%target-swift-flags', ''))
 
+# target_sdk_libcxx_path is empty when targeting non-Darwin platforms
+config.target_libcxx_path = config.target_sdk_libcxx_path
+
 if get_target_os() in ['linux-gnu']:
-    if os.path.exists('/usr/include/c++/v1') or os.path.exists('/usr/local/include/c++/v1'):
-        config.available_features.add('system_wide_libcxx')
+    for p in ['/usr/include/c++/v1', '/usr/local/include/c++/v1']:
+        if os.path.exists(p) or os.path.exists(p):
+            config.available_features.add('system_wide_libcxx')
+            config.target_libcxx_path = p
+
+if config.target_libcxx_path != '':
+    if os.path.exists(os.path.join(config.target_sdk_libcxx_path, "span")):
+        config.available_features.add('std_span')
+elif get_target_os() in ['windows-msvc']:
+    # We don't test on any versions of MSVC without std::span support.
+    # FIXME: figure out where to do lookup for C++ stdlib headers on Windows - we'll probably need it eventually
+    config.available_features.add('std_span')
 
 # Enable C++ interop when compiling Swift sources.
 config.substitutions.insert(0, ('%target-interop-build-swift',