The error messages are quite verbose and are best viewed in the try-job output but I'll include some excerpts here:

FAILED: obj/tools/battor_agent/battor_agent_lib/battor_agent.obj 
E:\b\c\goma_client/gomacc.exe ../../third_party/llvm-build/Release+Asserts/bin/clang-cl.exe /nologo /showIncludes  @obj/tools/battor_agent/battor_agent_lib/battor_agent.obj.rsp /c ../../tools/battor_agent/battor_agent.cc /Foobj/tools/battor_agent/battor_agent_lib/battor_agent.obj /Fd"obj/tools/battor_agent/battor_agent_lib_cc.pdb"
../../tools/battor_agent/battor_agent.cc(33,1):  error: use of undeclared identifier '__annotation'
TRACELOGGING_DEFINE_PROVIDER(



../../tools/battor_agent/battor_agent.cc(37,6):  error: expression result unused [-Werror,-Wunused-value]
    (0xc58f1405, 0x1112, 0x54dc, 0xad, 0xf1, 0xb8, 0xac, 0xb8, 0xc0, 0x50, 0xec));
     ^~~~~~~~~~


It's not clear whether the "result unused" warnings are a side-effect of the lack of support for __annotation or something separate.

(upstream: https://bugs.llvm.org/show_bug.cgi?id=30932)

It's more than just __annotation, there's also pre-processor incompatibility.

Here's how MSVC expands TRACELOGGING_DEFINE_PROVIDER after formatting:

static void __cdecl _TlgDefineProvider_annotation__Tlgg_hMyComponentProviderProv(
    void) {
  __annotation(L"_TlgDefineProvider:|"
               L"11"
               L"|"
               L"g_hMyComponentProvider"
               L"|"
               L"SimpleTraceLoggingProvider");
};
__pragma(execution_character_set(push, "UTF-8"))
    __pragma(pack(push, 1)) static struct {
  struct _TlgProviderMetadata_t _TlgProv;
  char _TlgName[sizeof("SimpleTraceLoggingProvider")];
} const __declspec(allocate(".rdata$zETW2")) __declspec(align(1))
    _Tlgg_hMyComponentProviderProv_Meta = {
        {_TlgBlobProvider3,
         {0x3970f9cf, 0x2c0c, 0x4f11, 0xb1, 0xcc, 0xe3, 0xa1, 0xe9, 0x95, 0x88,
          0x33},
         sizeof(_Tlgg_hMyComponentProviderProv_Meta) - 1 - 16},
        ("SimpleTraceLoggingProvider")};
__pragma(pack(pop)) __pragma(execution_character_set(
    pop)) static struct _TlgProvider_t _Tlgg_hMyComponentProviderProv = {
    0, &_Tlgg_hMyComponentProviderProv_Meta._TlgProv.RemainingSize,  0, 0, 0, 0,
    0, &_TlgDefineProvider_annotation__Tlgg_hMyComponentProviderProv};
extern const TraceLoggingHProvider g_hMyComponentProvider =
    &_Tlgg_hMyComponentProviderProv;

Here's how clang expands it:

static void __cdecl _TlgDefineProvider_annotation__Tlgg_hMyComponentProviderProv(
    void) {
  __annotation(L"_TlgDefineProvider:|"
               L"11"
               L"|"
               L"g_hMyComponentProvider"
               L"|"
               L"SimpleTraceLoggingProvider");
};
_TlgProviderStorage_imp0(_Tlgg_hMyComponentProviderProv,
                         "SimpleTraceLoggingProvider",
                         (0x3970f9cf, 0x2c0c, 0x4f11, 0xb1, 0xcc, 0xe3, 0xa1,
                          0xe9, 0x95, 0x88, 0x33),
                         1);
extern const TraceLoggingHProvider g_hMyComponentProvider =
    &_Tlgg_hMyComponentProviderProv;

"_TlgProviderStorage_imp0" is unexpanded.

https://bugs.llvm.org/show_bug.cgi?id=32021 is a better upstream bug. I reduced things a bit more over there.

Thanks for the update. I've confirmed that the newest version of clang-cl appears to handle __annotation directives, but the preprocessor incompatibilities are still a problem.