ThinLTO breaks MSVC `_imp_` symbols

[alexcrichton]

I believe this is a minimization of this reported issue:

$ cat bar.rs
pub static A: u32 = 43;

pub mod a {
    pub static A: u32 = 43;
}
$ cat foo.rs
extern crate bar;

fn main() {
    println!("{}", bar::A);
}
$ rustc +nightly bar.rs -C prefer-dynamic -Z thinlto -C codegen-units=1000 --crate-type dylib,rlib -O
$ rustc +nightly foo.rs -L . -Z thinlto -Z print-link-args
error: linking with `link.exe` failed: exit code: 1120
  |
  = note: "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Community\\VC\\Tools\\MSVC\\14.10.25017\\bin\\HostX64\\x64\\link.exe" "/NOLOGO" "/NXCOMPAT" "/LIBPATH:C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib" "foo.foo0-8cd878b7c8d78940dfe6697baf5b88ec-exe.rs.rust-cgu.o" "foo.foo1-8cd878b7c8d78940dfe6697baf5b88ec-exe.rs.rust-cgu.o" "/OUT:foo.exe" "foo.crate.allocator.rust-cgu.o" "/OPT:REF,NOICF" "/DEBUG" "/NATVIS:C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\etc\\liballoc.natvis" "/NATVIS:C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\etc\\libcore.natvis" "/LIBPATH:." "/LIBPATH:C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib" "C:\\msys64\\code\\rustfmt\\libbar.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\libstd-2c973d468aa52913.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\libpanic_unwind-10dcb2bb73715353.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\libunwind-eac2a22c7ef68dc5.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\liblibc-6232007b55cca9f2.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\liballoc_system-968b8d6a8dcae5d4.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\liballoc-bba7ff6cfd33a952.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\libstd_unicode-6001d57e726caccd.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\librand-d98f06e2ac3721a9.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\libcore-15daeee4c05dda88.rlib" "C:\\Users\\alex\\.rustup\\toolchains\\nightly-x86_64-pc-windows-msvc\\lib\\rustlib\\x86_64-pc-windows-msvc\\lib\\libcompiler_builtins-b02255a955089c1d.rlib" "advapi32.lib" "ws2_32.lib" "userenv.lib" "shell32.lib" "msvcrt.lib"
  = note: foo.foo0-8cd878b7c8d78940dfe6697baf5b88ec-exe.rs.rust-cgu.o : error LNK2019: unresolved external symbol __imp__ZN3bar1A17h8c3f932ba2a49b51E referenced in function _ZN3foo4main17hd45fff29dc617b3fE.llvm.C758FA11
          foo.exe : fatal error LNK1120: 1 unresolved externals


error: aborting due to previous error

[alexcrichton]

I've got a fix for this in the works.