llvm-build: Change CBackend and CppBackend to not use library_name. This will

llvm-build: Change CBackend and CppBackend to not use library_name. This will
change the generated library .a file name once we fully switch over, but
simplifies how we treat these targets without requiring more special casing
(since their library group name and the codegen library name currently map to
the same "llvm-config" style component name).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144251 91177308-0d34-0410-b5e6-96231b3b80d8