[clang] [Driver] Allow -e entry but reject -eentry (PR #72804)

[Fangrui Song via cfe-commits]

MaskRay wrote:

Driver used to accept `-e` but ignore it (at least for `Gnu.cpp`). My fcf8ada18f9cfb1261262e4b0399ae9ab40451f8 in 2020-July rendered `-e` (both `Joined` and `Separate`). I think if we impose a restriction that `-exxx` is unsupported, technically it is a breaking change but likely very few projects will notice it (and the issue is pretty straightforward to fix). 

This change will get us out of the business to support random `-exxx` link option that GCC driver happens to accept but we don't, like `-export-dynamic` #72781 .

https://github.com/llvm/llvm-project/pull/72804