Ninja: Update showIncludes prefix detection for clang-cl 18

Since commit LLVM/Clang commit `5523fefb01c2` ([clang][lex] Use
preferred path separator in includer-relative lookup, 2023-09-08), part
of the upcoming 18.x release, the output format of the showIncludes flag
has changed, where it now prints paths with double backslashes:

    Note: including file: .\\foo.h

Previously, we expected to see the path name in the form "./foo.h".

Extend the regex to match a path name starting with `.\`, in addition to
the existing matched patterns.

Modules/CMakeDetermineCompilerId.cmake

@@ -1173,7 +1173,7 @@ function(CMAKE_DETERMINE_MSVC_SHOWINCLUDES_PREFIX lang userflags)
     ENCODING AUTO # cl prints in console output code page
     )
   string(REPLACE "\n" "\n  " msg "  ${out}")
-  if(res EQUAL 0 AND "${out}" MATCHES "(^|\n)([^:\n][^:\n]+:[^:\n]*[^: \n][^: \n]:?[ \t]+)([A-Za-z]:\\\\|\\./|/)")
+  if(res EQUAL 0 AND "${out}" MATCHES "(^|\n)([^:\n][^:\n]+:[^:\n]*[^: \n][^: \n]:?[ \t]+)([A-Za-z]:\\\\|\\./|\\.\\\\|/)")
     set(CMAKE_${lang}_CL_SHOWINCLUDES_PREFIX "${CMAKE_MATCH_2}" PARENT_SCOPE)
     string(APPEND msg "\nFound prefix \"${CMAKE_MATCH_2}\"")
   else()

Tests/RunCMake/Ninja/RunCMakeTest.cmake

@@ -47,6 +47,7 @@ if(WIN32)
     set(maybe_MAKE_PROGRAM "-DRunCMake_MAKE_PROGRAM=${RunCMake_MAKE_PROGRAM}")
   endif()
   run_cmake_script(ShowIncludes-437-ClangCl-17 -DshowIncludes=${showIncludes} ${maybe_MAKE_PROGRAM})
+  run_cmake_script(ShowIncludes-437-ClangCl-18 -DshowIncludes=${showIncludes} ${maybe_MAKE_PROGRAM})
   run_cmake_script(ShowIncludes-437-English -DshowIncludes=${showIncludes} ${maybe_MAKE_PROGRAM})
   run_cmake_script(ShowIncludes-437-French -DshowIncludes=${showIncludes} ${maybe_MAKE_PROGRAM})
   run_cmake_script(ShowIncludes-437-German -DshowIncludes=${showIncludes} ${maybe_MAKE_PROGRAM})

Tests/RunCMake/Ninja/ShowIncludes-437-ClangCl-18-check.cmake

@@ -0,0 +1,3 @@
+# 'clang-cl /showIncludes' prefix for clang-cl >= 18.
+set(expect "Note: including file: ")
+include(${CMAKE_CURRENT_LIST_DIR}/ShowIncludes-check.cmake)

Tests/RunCMake/Ninja/ShowIncludes-437-ClangCl-18-stdout.txt

@@ -0,0 +1 @@
+-- showIncludes='Note: including file: '

Tests/RunCMake/Ninja/ShowIncludes-437-ClangCl-18.cmake

@@ -0,0 +1,3 @@
+set(CODEPAGE 437)
+set(VSLANG "clang-cl-18") # Special case for test, not a real VS value.
+include(${CMAKE_CURRENT_LIST_DIR}/ShowIncludes.cmake)

Tests/RunCMake/showIncludes.c

@@ -36,6 +36,14 @@ int main()
     }
   }
 
+  // clang-cl >= 18 (special case for test, not a real VS value).
+  if (strcmp(vslang, "clang-cl-18") == 0) {
+    if (cp == 437 || cp == 65001) {
+      printf("Note: including file: .\\\\foo.h\n");
+      return 0;
+    }
+  }
+
   // msvc-wine (special case for test, not a real VS value).
   if (strcmp(vslang, "msvc-wine") == 0) {
     if (cp == 437 || cp == 65001) {