[Cxx] Make C++ flags configurable for Linux SDK (#59764)

Allow Linux distributions to provide their own C++ flags to compile the
C++ overlay correctly. The default is kept the same for Ubuntu and
CentOS, but other distributions can provide other flags to use their own
distro GCC stdlibc++ or even libc++ if they choose.

This should not change the current compilation, but opens the door for
other maintainers to provide a different value that work on their
systems.

Author: drodriguez

cmake/modules/SwiftConfigureSDK.cmake

@@ -282,6 +282,21 @@ macro(configure_sdk_unix name architectures)
   endif()
   set(SWIFT_SDK_${prefix}_USE_ISYSROOT FALSE)
 
+  # 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`.
+  # FIXME: This is a compromise. The value might depend on the architecture
+  # but add_swift_target_library does not allow passing different values
+  # depending on the architecture, so having a single value is the only
+  # possibility right now.
+  set(SWIFT_SDK_${prefix}_CXX_OVERLAY_SWIFT_COMPILE_FLAGS
+      -Xcc --gcc-toolchain=/usr
+    CACHE STRING "Extra flags for compiling the C++ overlay")
+
   foreach(arch ${architectures})
     if("${prefix}" STREQUAL "ANDROID")
       swift_android_sysroot(android_sysroot)

stdlib/public/Cxx/std/CMakeLists.txt

@@ -143,12 +143,7 @@ add_swift_target_library(swiftstd ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_SDK_OVE
     -Xfrontend -module-interface-preserve-types-as-written
 
     SWIFT_COMPILE_FLAGS_LINUX
-    # 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
+    ${SWIFT_SDK_LINUX_CXX_OVERLAY_SWIFT_COMPILE_FLAGS}
 
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
     TARGET_SDKS ALL_APPLE_PLATFORMS LINUX