Fix Check failed: buffer.str().take_front(4) == kZtsPrefix on Windows.

[copybara-service]

Fix Check failed: buffer.str().take_front(4) == kZtsPrefix on Windows.

Importer::GetMangledName used the target platform's mangler to compute
the mangled name of a tag type. When targeting an MSVC-ABI platform
this produced a Microsoft-mangled name, and rs_bindings_from_cc
aborted on CHECK(buffer.str().take_front(4) == kZtsPrefix). The fix
is to always use Itanium mangling for Crubit's mangled_cc_name.

mangled_cc_name doesn't have to correspond to a real symbol, because
it is only used to construct Crubit-specific names. All three callers
should be therefore okay with an Itanium name on every target platform:

• Rust identifiers for C++ template specializations
  (__CcTemplateInst...) and C++ identifiers for virtual upcast
  thunks. The Microsoft scheme uses ?, @ and $, which are not
  valid in identifiers.
• #[cfi_encoding], which rustc defines in terms of the Itanium
  C++ ABI mangling on all target platforms. See also
  https://github.com/rust-lang/rust/blob/574ff7d98bd6d037e5236a8453029173b32631fd/compiler/rustc_sanitizers/src/cfi/typeid/mod.rs#L39-L55

This change adds a second, always-Itanium mangler that is used only for
tag type names. mangler_ keeps its role of producing real symbol
names for the target.

Also adds Platform::X86Windows so that the new regression test in
ir_from_cc_test.rs can ask for x86_64-pc-windows-msvc bindings.