Default rust.debug = true for compiler contribs

This profile has only undergone minimal tweaks since it was originally
drafted. I asked a number of compiler contributors and they said they
set rust.debug explicitly. This was even true for one contributor that
set `rust.debug` = false! Almost everyone seems slightly surprised that
`rust.debug = true` is not the default. Let reality match expectation.

src/bootstrap/defaults/config.compiler.toml

@@ -6,8 +6,9 @@ compiler-docs = true
 [rust]
 # This enables `RUSTC_LOG=debug`, avoiding confusing situations
 # where adding `debug!()` appears to do nothing.
+# It also improves the quality of information from backtraces, profiling, etc.
 # However, it makes running the compiler slightly slower.
-debug-logging = true
+debug = true
 # This greatly increases the speed of rebuilds, especially when there are only minor changes. However, it makes the initial build slightly slower.
 incremental = true
 # Print backtrace on internal compiler errors during bootstrap