[clang] Fix ClangScanDeps test for #61006.

[ingomueller-net]

Summary

This patch solves two problems with the recently added test, which
caused CI failure in an internal downstream system:

1. It connects subsequent RUN statements with && into a single statement; otherwise, the variable defined in the first of them is out of scope in the subsequent ones where it is used.
2. In our hermetic testing environment, the clang++ binary under test does not have access to standard include paths (/usr/include etc.), so the test fails because it does not find stddef.h. The patch thus changes the test to use a locally created a.h header file.

clang/test/ClangScanDeps/pr61006.cppm

 // The slash direction in linux and windows are different.
 // Also the command to create symbolic link is different.
 // UNSUPPORTED: system-windows
 //
 // RUN: rm -fr %t
 // RUN: mkdir -p %t
 // RUN: split-file %s %t
 //
-// RUN: EXPECTED_RESOURCE_DIR=`%clang -print-resource-dir`
-// RUN: ln -s %clang++ %t/clang++
-// RUN: sed "s|EXPECTED_RESOURCE_DIR|$EXPECTED_RESOURCE_DIR|g; s|DIR|%/t|g" %t/P1689.json.in > %t/P1689.json
-// RUN: clang-scan-deps -compilation-database %t/P1689.json -format=p1689 | FileCheck %t/a.cpp -DPREFIX=%/t
+// RUN: EXPECTED_RESOURCE_DIR=`%clang -print-resource-dir` && \
+// RUN: ln -s %clang++ %t/clang++ && \
+// RUN: sed "s|EXPECTED_RESOURCE_DIR|$EXPECTED_RESOURCE_DIR|g; s|DIR|%/t|g" %t/P1689.json.in > %t/P1689.json && \
+// RUN: clang-scan-deps -compilation-database %t/P1689.json -format=p1689 | FileCheck %t/a.cpp -DPREFIX=%/t && \
 // RUN: clang-scan-deps -format=p1689 \
 // RUN:   -- %t/clang++ -std=c++20 -c -fprebuilt-module-path=%t %t/a.cpp -o %t/a.o \
 // RUN:      -resource-dir $EXPECTED_RESOURCE_DIR | FileCheck %t/a.cpp -DPREFIX=%/t
 
 //--- P1689.json.in
 [
 {
   "directory": "DIR",
   "command": "DIR/clang++ -std=c++20 -c -fprebuilt-module-path=DIR DIR/a.cpp -o DIR/a.o -resource-dir EXPECTED_RESOURCE_DIR",
   "file": "DIR/a.cpp",
   "output": "DIR/a.o"
 }
 ]
 
 //--- a.cpp
-#include <stddef.h>
+#include "a.h"
 import b;
 
 // CHECK: {
 // CHECK-NEXT:   "revision": 0,
 // CHECK-NEXT:   "rules": [
 // CHECK-NEXT:     {
 // CHECK-NEXT:       "primary-output": "[[PREFIX]]/a.o",
 // CHECK-NEXT:       "requires": [
 // CHECK-NEXT:         {
 // CHECK-NEXT:           "logical-name": "b"
 // CHECK-NEXT:         }
 // CHECK-NEXT:       ]
 // CHECK-NEXT:     }
 // CHECK-NEXT:   ],
 // CHECK-NEXT:   "version": 1
 // CHECK-NEXT: }
+
+//--- a.h