Windows needs explicit exports of functions from DLLs but LLVM does not mention
any of its symbols as being export-able from a DLL. The compiler, however,
relies on being able to use LLVM symbols across DLL boundaries so we need to
force many of LLVM's symbols to be exported from `rustc_llvm.dll`. This commit
adds support for generation of a `rustc_llvm.def` file which is passed along to
the linker when generating `rustc_llvm.dll` which should keep all these symbols
exportable and usable.
b538189ba0