check host's libstdc++ version when using ci llvm

If the host's libstdc++ version is too old using ci-llvm may result in an ABI mismatch
between the local libstdc++ and libLLVM.so. This PR adds a sanity check to immediately fail
at the beginning of the bootstrap before starting the actual build. I am not sure if '8.0.0'
is the best threshold, but it should be a good start and we can increase it anytime if needed.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/ci-llvm-libstdcpp-version-threshold

@@ -0,0 +1 @@
+8.0.0

src/bootstrap/src/core/sanity.rs

@@ -99,6 +99,22 @@ pub fn check(build: &mut Build) {
         cmd_finder.must_have("git");
     }
 
+    if !build.config.dry_run() && build.config.llvm_from_ci {
+        let cxx = build.cxx(build.build).unwrap();
+        let mut cxx_cmd = Command::new(cxx);
+        cxx_cmd.arg("-dumpversion");
+
+        let current_libstdcpp_v = semver::Version::parse(output(&mut cxx_cmd).trim()).unwrap();
+        let libstdcpp_v_threshold = semver::Version::parse(include_str!("../../ci-llvm-libstdcpp-version-threshold")).unwrap();
+
+        if current_libstdcpp_v.lt(&libstdcpp_v_threshold) {
+            eprintln!("\nCurrent libstdc++ version is '{current_libstdcpp_v}', which is too old and below the required threshold of '{libstdcpp_v_threshold}'.");
+            // In case the user has a custom setup that includes a recent libstdc++ with an old c++ compiler, this information might be useful to them.
+            eprintln!("If you think this is a mistake, you can bypass this sanity check by setting 'BOOTSTRAP_SKIP_TARGET_SANITY' env variable to '1'.");
+            crate::exit!(1);
+        }
+    }
+
     // We need cmake, but only if we're actually building LLVM or sanitizers.
     let building_llvm = build
         .hosts