MFC 309775,312897: Don't delete /usr/bin/ld if it is lld.

309775:
Keep /usr/bin/ld for WITHOUT_BINUTILS but WITH_LLD_AS_LD

312897:
Rename LLD_AS_LD to LLD_IS_LD, for consistency with CLANG_IS_CC

An additional case missed in r312855

Reported by: Mark Millard