[clang] [Clang][Driver] Warn about `-c/-S` with `-fsyntax-only` (PR #98607)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-win` running on `sie-win-worker` while building `clang` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/2069

Here is the relevant piece of the build log for the reference:
```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: Driver/warn-fsyntax-only.c' FAILED ********************
Exit Code: 2

Command Output (stdout):
--
# RUN: at line 1
z:\b\llvm-clang-x86_64-sie-win\build\bin\clang.exe -fsyntax-only -E Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c 2>&1 | z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c --check-prefix=CHECK-PP
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\clang.exe' -fsyntax-only -E 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c'
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe' 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c' --check-prefix=CHECK-PP
# RUN: at line 2
z:\b\llvm-clang-x86_64-sie-win\build\bin\clang.exe -fsyntax-only -S Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c 2>&1 | z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c --check-prefix=CHECK-ASM
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\clang.exe' -fsyntax-only -S 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c'
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe' 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c' --check-prefix=CHECK-ASM
# .---command stderr------------
# | FileCheck error: '<stdin>' is empty.
# | FileCheck command line:  z:\b\llvm-clang-x86_64-sie-win\build\bin\filecheck.exe Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\Driver\warn-fsyntax-only.c --check-prefix=CHECK-ASM
# `-----------------------------
# error: command failed with exit status: 2

--

********************


```

https://github.com/llvm/llvm-project/pull/98607