[clang] [Clang] Add 'CLANG_ALLOW_IMPLICIT_RPATH' to enable toolchain use of -rpath (PR #82004)

[Shilei Tian via cfe-commits]

shiltian wrote:

IMHO I prefer to ask/request users to do the right thing. Vendors are totally free to do whatever to be convenient for their customers via their compiler wrappers/drivers, but for the community version, following the convention would be good.

https://github.com/llvm/llvm-project/pull/82004