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

1 files changed, 9 insertions, 0 deletions

diff --git a/src/test/run-make-fulldeps/instrument-coverage/prettify_json.py b/src/test/run-make-fulldeps/instrument-coverage/prettify_json.py
new file mode 100644
index 00000000000..ed9279841f7
--- /dev/null
+++ b/src/test/run-make-fulldeps/instrument-coverage/prettify_json.py
@@ -0,0 +1,9 @@
+#!/usr/bin/env python
+
+import sys
+import json
+
+# Try to decode line in order to ensure it is a valid JSON document
+for line in sys.stdin:
+    parsed = json.loads(line)
+    print (json.dumps(parsed, indent=2, separators=(',', ': '), sort_keys=True))