Allow emission of names that start with an underscore. This is needed to

Allow emission of names that start with an underscore.  This is needed to
interface with code that uses symbols in the ansi-c protected namespace.  In
most cases this comes from system header files, such as stdio.h.  In particular,
without this change, a reference to the __iob symbol is mangled into ll_iob,
which is not resolved by libc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4165 91177308-0d34-0410-b5e6-96231b3b80d8