Add an alternate --demangle mode to coverage-dump

The coverage-dump tool already needs `rustc_demangle` for its own purposes, so
the amount of extra code needed for a demangle mode is very small.

src/tools/coverage-dump/README.md

@@ -6,3 +6,8 @@ The output format is mostly arbitrary, so it's OK to change the output as long
 as any affected tests are also re-blessed. However, the output should be
 consistent across different executions on different platforms, so avoid
 printing any information that is platform-specific or non-deterministic.
+
+## Demangle mode
+
+When run as `coverage-dump --demangle`, this tool instead functions as a
+command-line demangler that can be invoked by `llvm-cov`.

src/tools/coverage-dump/src/main.rs

@@ -7,6 +7,13 @@ fn main() -> anyhow::Result<()> {
 
     let args = std::env::args().collect::<Vec<_>>();
 
+    // The coverage-dump tool already needs `rustc_demangle` in order to read
+    // coverage metadata, so it's very easy to also have a separate mode that
+    // turns it into a command-line demangler for use by coverage-run tests.
+    if &args[1..] == &["--demangle"] {
+        return demangle();
+    }
+
     let llvm_ir_path = args.get(1).context("LLVM IR file not specified")?;
     let llvm_ir = std::fs::read_to_string(llvm_ir_path).context("couldn't read LLVM IR file")?;
 
@@ -15,3 +22,15 @@ fn main() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+fn demangle() -> anyhow::Result<()> {
+    use std::fmt::Write as _;
+
+    let stdin = std::io::read_to_string(std::io::stdin())?;
+    let mut output = String::with_capacity(stdin.len());
+    for line in stdin.lines() {
+        writeln!(output, "{:#}", rustc_demangle::demangle(line))?;
+    }
+    print!("{output}");
+    Ok(())
+}