[clang] Fix ClangScanDeps test for #61006.

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.

Differential Revision: https://reviews.llvm.org/D157817