[libc++][C++20 modules] Tests no wchar_t build.

[Mordante]

Summary

Depends on D158358

libcxx/cmake/caches/Generic-no-wide-characters.cmake

+set(LIBCXX_ENABLE_STD_MODULES ON CACHE BOOL "") # TODO MODULES Remove when enabled automatically.
 set(LIBCXX_ENABLE_WIDE_CHARACTERS OFF CACHE BOOL "")

libcxx/modules/std/cstdlib.inc

export namespace std {
   using std::strtof;
   using std::strtol;
   using std::strtold;
   using std::strtoll;
   using std::strtoul;
   using std::strtoull;
 
   // [c.mb.wcs], multibyte / wide string and character conversion functions
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
   using std::mblen;
+#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
   using std::mbstowcs;
   using std::mbtowc;
   using std::wcstombs;
   using std::wctomb;
 #endif
   // [alg.c.library], C standard library algorithms
   using std::bsearch;
   using std::qsort;

libcxx/utils/ci/buildkite-pipeline.yml

- label: "No Unicode"
       timeout_in_minutes: 120
 
     - label: "No wide characters"
       command: "libcxx/utils/ci/run-buildbot generic-no-wide-characters"
       artifact_paths:
         - "**/test-results.xml"
         - "**/*.abilist"
       env:
-          CC: "clang-${LLVM_HEAD_VERSION}"
-          CXX: "clang++-${LLVM_HEAD_VERSION}"
+          # Note: Modules require and absolute path for clang-scan-deps
+          # https://github.com/llvm/llvm-project/issues/61006
+          CC: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang"
+          CXX: "/usr/lib/llvm-${LLVM_HEAD_VERSION}/bin/clang++"
+          CMAKE: "/opt/bin/cmake"
           ENABLE_CLANG_TIDY: "On"
       agents:
         queue: "libcxx-builders"
         os: "linux"
       retry:
         automatic:
           - exit_status: -1  # Agent was lost
             limit: 2