[cxx-interop] Enable C++ stdlib overlay on Linux

Author: egorzhdan

stdlib/public/Cxx/CMakeLists.txt

@@ -139,7 +139,13 @@ add_swift_target_library(swiftstd ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVE
     SWIFT_COMPILE_FLAGS ${SWIFT_RUNTIME_SWIFT_COMPILE_FLAGS} ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -Xfrontend -enable-experimental-cxx-interop
     -Xfrontend -module-interface-preserve-types-as-written
-
-    LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}" -lc++
-    TARGET_SDKS ALL_APPLE_PLATFORMS # TODO: support other platforms as well
+    # GCC on Linux is usually located under `/usr`.
+    # However, Ubuntu 20.04 ships with another GCC installation under `/`, which does not include libstdc++.
+    # Swift build scripts pass `--sysroot=/` to Clang. By default, Clang tries to find GCC installation under sysroot,
+    # and if it doesn't exist, under `{sysroot}/usr`. On Ubuntu 20.04 and newer, it attempts to use the GCC without the
+    # C++ stdlib, which causes a build failure. To fix that, we tell Clang explicitly to use GCC from `/usr`.
+    -Xcc --gcc-toolchain=/usr
+
+    LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
+    TARGET_SDKS ALL_APPLE_PLATFORMS LINUX # TODO: support other platforms as well
     INSTALL_IN_COMPONENT sdk-overlay)

test/Interop/Cxx/stdlib/overlay/string.swift

@@ -1,7 +1,7 @@
 // RUN: %target-run-simple-swift(-Xfrontend -enable-experimental-cxx-interop)
 //
 // REQUIRES: executable_test
-// REQUIRES: OS=macosx
+// REQUIRES: OS=macosx || OS=linux-gnu
 
 import StdlibUnittest
 import std