tracing log along diff crates

Author: bvanjoi

src/tracing.md

@@ -144,6 +144,10 @@ $ RUSTC_LOG=debug rustc +stage1 my-file.rs 2>all-log
 # compilers.
 $ RUSTC_LOG=rustc_codegen_ssa=info rustc +stage1 my-file.rs
 
+# This will show the all tracing calls under `rustc_codegen_ssa` and
+# `rustc_resolve`.
+$ RUSTC_LOG=rustc_codegen_ssa,rustc_resolve rustc +stage1 my-file.rs
+
 # This will show the output of all `info!` calls made by rustdoc
 # or any rustc library it calls.
 $ RUSTDOC_LOG=info rustdoc +stage1 my-file.rs