Auto merge of #74091 - richkadel:llvm-coverage-map-gen-4, r=tmandry

Auto merge of #74091 - richkadel:llvm-coverage-map-gen-4, r=tmandry

Generating the coverage map

@tmandry @wesleywiser

rustc now generates the coverage map and can support (limited)
coverage report generation, at the function level.

Example commands to generate a coverage report:
```shell
$ BUILD=$HOME/rust/build/x86_64-unknown-linux-gnu
$ $BUILD/stage1/bin/rustc -Zinstrument-coverage \
$HOME/rust/src/test/run-make-fulldeps/instrument-coverage/main.rs
$ LLVM_PROFILE_FILE="main.profraw" ./main
called
$ $BUILD/llvm/bin/llvm-profdata merge -sparse main.profraw -o main.profdata
$ $BUILD/llvm/bin/llvm-cov show --instr-profile=main.profdata main
```
![rust coverage report only 20200706](https://user-images.githubusercontent.com/3827298/86697299-1cbe8f80-bfc3-11ea-8955-451b48626991.png)

r? @wesleywiser

Rust compiler MCP rust-lang/compiler-team#278
Relevant issue: #34701 - Implement support for LLVMs code coverage instrumentation