Make [rust] use-lld=true work on windows

Before, it would fail with "error: ignoring unknown argument '-Wl,--icf=all'"

src/bootstrap/compile.rs

@@ -658,7 +658,12 @@ impl Step for Rustc {
 
         // With LLD, we can use ICF (identical code folding) to reduce the executable size
         // of librustc_driver/rustc and to improve i-cache utilization.
-        if builder.config.use_lld {
+        //
+        // -Wl,[link options] doesn't work on MSVC. However, /OPT:ICF (technically /OPT:REF,ICF)
+        // is already on by default in MSVC optimized builds, which is interpreted as --icf=all:
+        // https://github.com/llvm/llvm-project/blob/3329cec2f79185bafd678f310fafadba2a8c76d2/lld/COFF/Driver.cpp#L1746
+        // https://github.com/rust-lang/rust/blob/f22819bcce4abaff7d1246a56eec493418f9f4ee/compiler/rustc_codegen_ssa/src/back/linker.rs#L827
+        if builder.config.use_lld && !compiler.host.contains("msvc") {
             cargo.rustflag("-Clink-args=-Wl,--icf=all");
         }