Auto merge of #114843 - Zalathar:test-coverage-map, r=oli-obk

coverage: Explicitly test the coverage maps produced by codegen/LLVM

Our existing coverage tests verify the output of end-to-end coverage reports, but we don't have any way to test the specific mapping information (code regions and their associated counters) that are emitted by `rustc_codegen_llvm` and LLVM. That makes it harder to to be confident in changes that would modify those mappings (whether deliberately or accidentally).

This PR addresses that by adding a new `coverage-map` test suite that does the following:
- Compiles test files to LLVM IR assembly (`.ll`)
- Feeds those IR files to a custom tool (`src/tools/coverage-dump`) that extracts and decodes coverage mappings, and prints them in a more human-readable format
- Checks the output of that tool against known-good snapshots

---

I recommend excluding the last commit while reviewing the main changes, because that last commit is just ~40 test files copied over from `tests/run-coverage`, plus their blessed coverage-map snapshots and a readme file. Those snapshots aren't really intended to be checked by hand; they're mostly there to increase the chances that an unintended change to coverage maps will be observable (even if it requires relatively specific circumstances to manifest).

1 files changed, 9 insertions, 0 deletions

diff --git a/tests/coverage-map/if.rs b/tests/coverage-map/if.rs
new file mode 100644
index 00000000000..ed3f69bdc98
--- /dev/null
+++ b/tests/coverage-map/if.rs
@@ -0,0 +1,9 @@
+// compile-flags: --edition=2021
+
+fn main() {
+    let cond = std::env::args().len() == 1;
+    if cond {
+        println!("true");
+    }
+    println!("done");
+}