[WIP] C++ code started to create and publish coverage map

Author: richkadel

src/librustc_llvm/build.rs

@@ -169,6 +169,7 @@ fn main() {
     cfg.file("../rustllvm/PassWrapper.cpp")
         .file("../rustllvm/RustWrapper.cpp")
         .file("../rustllvm/ArchiveWrapper.cpp")
+        .file("../rustllvm/CoverageMappingWrapper.cpp")
         .file("../rustllvm/Linker.cpp")
         .cpp(true)
         .cpp_link_stdlib(None) // we handle this below