Tweaks for setting CMAKE_LINKER to lld-link

- Just look for "lld-link", not "lld-link.exe".
  llvm/cmake/platforms/WinMsvc.cmake for example sets CMAKE_LINKER to
  lld-link without .exe

- Stop passing -gwarf to the compiler in sanitizer options when lld is
  enabled -- there's no reason to use different debug information keyed
  off the linker. (If this was for MinGW, we should check for that
  instead.)

Differential Revision: https://reviews.llvm.org/D62111

llvm-svn: 361214