[clang] drop buggy use of `-serialize-diagnostics` flag

[ashay-github]

Summary

The -serialize-diagnostics flag requires a filename to be passed
immediately after it, but the filename argument was skipped in the
P1689.cppm clang test. This caused the code to incorrectly consume the
argument that followed as the dignostics file.

Since the -serialize-diagnostics flag isn't needed for this test to
work, this patch removes it instead of passing a file argument.

[ChuanqiXu]

Thanks!

[ashay-github]

Rebased with main branch.

[MaskRay]

For pure test updates, best to include [test] in the subject :)

[ashay-github]

For pure test updates, best to include [test] in the subject :)

Ah, sorry! Thanks for letting me know, I'll keep that in mind for the future.

clang/test/ClangScanDeps/P1689.cppm

 // RUN:      -MT %t/impl_part.o.ddi -MD -MF %t/impl_part.dep
 // RUN:   cat %t/impl_part.dep | FileCheck %t/impl_part.cppm -DPREFIX=%/t --check-prefix=CHECK-MAKE
 //
 // Check that we can generate multiple make-style dependency information with compilation database.
 // RUN: cat %t/P1689.dep | FileCheck %t/Checks.cpp -DPREFIX=%/t --check-prefix=CHECK-MAKE
 //
 // Check that we can mix the use of -format=p1689 and -fmodules.
 // RUN: clang-scan-deps -format=p1689 \
-// RUN:   -- %clang++ -std=c++20 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache -serialize-diagnostics -c %t/impl_part.cppm -o %t/impl_part.o \
+// RUN:   -- %clang++ -std=c++20 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t/cache -c %t/impl_part.cppm -o %t/impl_part.o \
 // RUN:   | FileCheck %t/impl_part.cppm -DPREFIX=%/t
 
 //--- P1689.json.in
 [
 {
   "directory": "DIR",
   "command": "clang++ -std=c++20 DIR/M.cppm -c -o DIR/M.o -MT DIR/M.o.ddi -MD -MF DIR/P1689.dep",
   "file": "DIR/M.cppm",