build-compiler-rt: Stop providing asan in msvcrt builds

See 0b296aefc0 for context, where
we stopped testing asan in msvcrt builds - see discussion at
https://github.com/llvm/llvm-project/pull/120397,
https://github.com/mstorsjo/llvm-mingw/issues/224 and
https://github.com/mstorsjo/llvm-mingw/issues/552.

This should make it clearer that asan isn't supported in msvcrt
builds, instead of silently just providing files that are known
not to work.

See 4718b81419 for earlier context
for where we remove the asan runtimes in armv7/aarch64 builds,
where the files do get built, but they don't work properly yet
(even on UCRT).

build-compiler-rt.sh

@@ -82,6 +82,18 @@ else
     esac
 fi
 
+cat<<EOF > is-ucrt.c
+#include <corecrt.h>
+#if !defined(_UCRT)
+#error not ucrt
+#endif
+EOF
+ANY_ARCH=$(echo $ARCHS | awk '{print $1}')
+if $ANY_ARCH-w64-mingw32-gcc$TOOLEXT -E is-ucrt.c > /dev/null 2>&1; then
+    IS_UCRT=1
+fi
+rm -f is-ucrt.c
+
 cd llvm-project/compiler-rt
 
 INSTALL_PREFIX="$CLANG_RESOURCE_DIR"
@@ -162,19 +174,26 @@ for arch in $ARCHS; do
     cmake --install . --prefix "$INSTALL_PREFIX"
     mkdir -p "$PREFIX/$arch-w64-mingw32/bin"
     if [ -n "$SANITIZERS" ]; then
-        case $arch in
-        aarch64)
-            # asan doesn't work on aarch64 or armv7; make this clear by omitting
-            # the installed files altogether.
-            rm -f "$INSTALL_PREFIX/lib/windows/libclang_rt.asan"*aarch64*
-            ;;
-        armv7)
-            rm -f "$INSTALL_PREFIX/lib/windows/libclang_rt.asan"*arm*
-            ;;
-        *)
-            mv "$INSTALL_PREFIX/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
-            ;;
-        esac
+        if [ -z "$IS_UCRT" ]; then
+            # For msvcrt builds, remove the asan files; asan doesn't work
+            # properly on top of msvcrt, only on top of UCRT. Make this clear
+            # by omitting the installed files altogether.
+            rm -f "$INSTALL_PREFIX/lib/windows/libclang_rt.asan"*
+        else
+            case $arch in
+            aarch64)
+                # asan doesn't work on aarch64 or armv7; make this clear by omitting
+                # the installed files altogether.
+                rm -f "$INSTALL_PREFIX/lib/windows/libclang_rt.asan"*aarch64*
+                ;;
+            armv7)
+                rm -f "$INSTALL_PREFIX/lib/windows/libclang_rt.asan"*arm*
+                ;;
+            *)
+                mv "$INSTALL_PREFIX/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
+                ;;
+            esac
+        fi
     fi
     cd ..
 done