Revert use of Clang config files

This reverts commit 49db4a3a15,
9f1f0104f1,
4f95c14adf,
c34dc64028,
570fdcd251,
4bff0298c5 and
6939aee0c0.

When relying on Clang config files, two issues appeared:

- Running the compiler-rt tests fails, because the compiler-rt tests
  disable any default config files, breaking the compiler
  in this case:
  https://github.com/llvm/llvm-project/blob/llvmorg-19.1.1/compiler-rt/test/lit.common.cfg.py#L983

- Wine fails to build using llvm-mingw as cross compiler, because
  Wine adds the --no-default-config flag to Clang while compiling,
  if supported:
  https://gitlab.winehq.org/wine/wine/-/blob/wine-9.19/configure.ac?ref_type=tags#L866-868
  This causes llvm-mingw to omit the -rtlib=compiler-rt option
  (specified in a config file), making the compiler look for libgcc
  instead. Doing a check to see that linking succeeds doesn't necessarily
  help here - such a check already exists, but it tests linking with
  -nostdlib -nodefaultlibs:
  https://gitlab.winehq.org/wine/wine/-/blob/wine-9.19/aclocal.m4?ref_type=tags#L193-L206

build-llvm.sh

@@ -242,6 +242,17 @@ if [ -n "$COMPILER_LAUNCHER" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_CXX_COMPILER_LAUNCHER=$COMPILER_LAUNCHER"
 fi
 
+if [ -n "$TARGET_WINDOWS" ]; then
+    # Custom, llvm-mingw specific defaults. We normally set these in
+    # the frontend wrappers, but this makes sure they are enabled by
+    # default if that wrapper is bypassed as well.
+    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_RTLIB=compiler-rt"
+    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_UNWINDLIB=libunwind"
+    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_CXX_STDLIB=libc++"
+    CMAKEFLAGS="$CMAKEFLAGS -DCLANG_DEFAULT_LINKER=lld"
+    CMAKEFLAGS="$CMAKEFLAGS -DLLD_DEFAULT_LD_LLD_IS_MINGW=ON"
+fi
+
 if [ -n "$LTO" ]; then
     CMAKEFLAGS="$CMAKEFLAGS -DLLVM_ENABLE_LTO=$LTO"
 fi