xcbuild: assorted fixes and cleanups

This is in preparation for the LLVM 4 upgrade (which gets more strict
about e.g., return false in xcbuild itself) and also for using xcbuild
more extensively in the Darwin stdenv bootstrap process, which is why I
killed the unnecessary gcc dependency in the toolchain. llvm-cov pretends
to be gcov anyway, so we're fine.

pkgs/development/compilers/llvm/3.7/clang/default.nix

@@ -42,6 +42,7 @@ let
     passthru = {
       lib = self; # compatibility with gcc, so that `stdenv.cc.cc.lib` works on both
       isClang = true;
+      inherit llvm;
     } // stdenv.lib.optionalAttrs stdenv.isLinux {
       inherit gcc;
     };

pkgs/development/compilers/llvm/3.8/clang/default.nix

@@ -53,6 +53,7 @@ let
     passthru = {
       lib = self; # compatibility with gcc, so that `stdenv.cc.cc.lib` works on both
       isClang = true;
+      inherit llvm;
     } // stdenv.lib.optionalAttrs stdenv.isLinux {
       inherit gcc;
     };

pkgs/development/compilers/llvm/3.9/clang/default.nix

@@ -42,6 +42,7 @@ let
     passthru = {
       lib = self; # compatibility with gcc, so that `stdenv.cc.cc.lib` works on both
       isClang = true;
+      inherit llvm;
     } // stdenv.lib.optionalAttrs stdenv.isLinux {
       inherit gcc;
     };

pkgs/development/compilers/llvm/4/clang/default.nix

@@ -53,6 +53,7 @@ let
     passthru = {
       lib = self; # compatibility with gcc, so that `stdenv.cc.cc.lib` works on both
       isClang = true;
+      inherit llvm;
     } // stdenv.lib.optionalAttrs stdenv.isLinux {
       inherit gcc;
     };