rustc: Link statically to the MSVCRT

This commit changes all MSVC rustc binaries to be compiled with
`-C target-feature=+crt-static` to link statically against the MSVCRT instead of
dynamically (as it does today). This also necessitates compiling LLVM in a
different fashion, ensuring it's compiled with `/MT` instead of `/MD`.

cc #37406

Author: alexcrichton

src/bootstrap/bin/rustc.rs

@@ -205,6 +205,11 @@ fn main() {
                 }
             }
         }
+
+        if target.contains("pc-windows-msvc") {
+            cmd.arg("-Z").arg("unstable-options");
+            cmd.arg("-C").arg("target-feature=+crt-static");
+        }
     }
 
     if verbose > 1 {

src/bootstrap/native.rs

@@ -99,6 +99,12 @@ pub fn llvm(build: &Build, target: &str) {
        .define("LLVM_TARGET_ARCH", target.split('-').next().unwrap())
        .define("LLVM_DEFAULT_TARGET_TRIPLE", target);
 
+    if target.contains("msvc") {
+        cfg.define("LLVM_USE_CRT_DEBUG", "MT");
+        cfg.define("LLVM_USE_CRT_RELEASE", "MT");
+        cfg.define("LLVM_USE_CRT_RELWITHDEBINFO", "MT");
+    }
+
     if target.starts_with("i686") {
         cfg.define("LLVM_BUILD_32_BITS", "ON");
     }