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

wrappers/clang-scan-deps-wrapper.c

@@ -63,7 +63,7 @@ int _tmain(int argc, TCHAR *argv[]) {
         if (sep)
             *sep = '\0';
         TCHAR *dash = _tcsrchr(cmd_exe, '-');
-        int has_target = 0;
+        const TCHAR *target = NULL;
         if (dash) {
             *dash = '\0';
             const TCHAR *cmd_exe_suffix = dash + 1;
@@ -76,15 +76,14 @@ int _tmain(int argc, TCHAR *argv[]) {
                 !_tcscmp(cmd_exe_suffix, _T("cc")) ||
                 !_tcscmp(cmd_exe_suffix, _T("c99")) ||
                 !_tcscmp(cmd_exe_suffix, _T("c11"))) {
-                has_target = 1;
+                target = cmd_exe;
             }
         }
-        const TCHAR *target = NULL;
 #ifdef _WIN32
         // On Windows, we want to set our default target even if no target
         // was found in cmd_exe, as we want to support running with a foreign
         // clang-scan-deps-real.exe binary, that could have any default.
-        if (!has_target)
+        if (!target)
             target = _T(DEFAULT_TARGET);
 #endif
 
@@ -93,6 +92,7 @@ int _tmain(int argc, TCHAR *argv[]) {
             // -target after cmd_exe.
             exec_argv[arg++] = _T("-target");
             exec_argv[arg++] = target;
+            exec_argv[arg++] = _T("-stdlib=libc++");
         }
     }