[PECOFF] Find symbols with @number suffix for dllexported symbols

As written in the comment in this patch, symbol names specified with
/export option is resolved in a special way; for /export:foo, linker
finds a foo@<number> symbol if such symbols exists.

On Windows, a function in stdcall calling convention is mangled with
a leading underscore and following "@" and numbers. This name
mangling is kind of automatic, so you can sometimes omit _ and @number
when specifying a symbol. /export option is that case.

Previously, if a file in an archive file foo.lib provides a symbol
_fn@8, and /export:fn is specified, LLD failed to resolve the symbol.
It only tried to find _fn, and failed to find _fn@8. With this patch,
_fn@8 will be searched on the second iteration.

Differential Revision: http://reviews.llvm.org/D3736

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@208754 91177308-0d34-0410-b5e6-96231b3b80d8