For background, see https://bugs.chromium.org/p/chromium/issues/detail?id=585930
Upstream bug: https://llvm.org/bugs/show_bug.cgi?id=11740

To reproduce:

$ cat conftest.c 
void foo() {}
$ x86_64-cros-linux-gnu-clang -g -S -o conftest.s conftest.c
$ x86_64-cros-linux-gnu-clang -g -c -o conftest.o conftest.s
conftest.s:8:2: error: input can't have .file dwarf directives when -g is used to generate dwarf debug info for assembly code
        .file   1 "conftest.c"
        ^
conftest.s:8:8: error: file number already allocated
        .file   1 "conftest.c"
                ^

Removing "-g" flag from clang command lines "fixes" the problem.