rustc_codegen_llvm: rewrite debuginfo::get_function_signature to use FnAbi.
rustc_codegen_llvm: rewrite debuginfo::get_function_signature to use FnAbi.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust