[PECOFF] Define implicit symbols for exported ones.

This patch is to fix a compatibility issue with MSVC link.exe as to
use of dllexported symbols inside DLL.

A DLL exports two symbols for a function. One is non-decorated one,
and the other is with __imp_ prefix. The former is a function that
you can directly call, and the latter is a pointer to the function.
These dllexported symbols are created by linker for programs that
link against the DLL. So, I naturally believed that __imp_ symbols
become available when you once create a DLL and link against it, but
they don't exist until then. And that's not true.

MSVC link.exe is smart enough to allow users to use __imp_ symbols
locally. That is, if a symbol is specified with /export option, it
implicitly creates a new symbol with __imp_ prefix as a pointer to
the exported symbol. This feature allows the following program to
be linked and run, although _imp__hello is not defined in this code.

  #include <stdio.h>

  __declspec(dllexport)
  void hello(void) { printf("Hello\n"); }

  extern void (*_imp__hello)(void);

  int main() {
    _imp__hello();
    return 0;
  }

MSVC link.exe prints out the following warning when linking it.

  LNK4217: locally defined symbol _hello imported in function _main

Using __imp_ symbols locally is I think not a good coding style. One
should just take an address using "&" operator rather than appending
__imp_ prefix. However, there are programs in the wild that depends
on this link.exe's behavior, so we need this feature.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@207141 91177308-0d34-0410-b5e6-96231b3b80d8