MSAN is saying std::cout is uninitialized

[ghost]

vm-kernel-msan-linux-release-x64 is currently failing with:

==11039==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7fa9b26ae04c in std::__2::basic_ostream<char, std::__2::char_traits<char> >& std::__2::__put_character_sequence<char, std::__2::char_traits<char> >(std::__2::basic_ostream<char, std::__2::char_traits<char> >&, char const*, unsigned long) ../../out/ReleaseMSANX64/../../buildtools/linux-x64/clang/bin/../include/c++/v1/ostream:727:13
    #1 0x7fa9b26a55ae in std::__2::basic_ostream<char, std::__2::char_traits<char> >& std::__2::operator<<<std::__2::char_traits<char> >(std::__2::basic_ostream<char, std::__2::char_traits<char> >&, char const*) ../../out/ReleaseMSANX64/../../buildtools/linux-x64/clang/bin/../include/c++/v1/ostream:869:12
    #2 0x7fa9b26a55ae in SumVeryManySmallInts ../../out/ReleaseMSANX64/../../runtime/bin/ffi_test/ffi_test_functions.cc:348:13

Which AFAICT points to std::cout: google/sanitizers#712.

The suggested fix is that libc++ needs to be built with MSan.

I do however not know if this is an issue with Dart, FFI, or something else.

[ghost]

[@dcharkes for FFI]

[dcharkes]

Thanks for reporting.

Duplicate of #43075.