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

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.

Differential Revision: https://reviews.llvm.org/D144934