[llvm] [cmake] Pass -resource-dir to external projects as CMAKE_<LANG>_FLAGS_INIT instead of as a force override (PR #87330)

[Raul Tambre via llvm-commits]

tambry wrote:

Ping.  
There doesn't really seem to be anyone more qualified than you two and I'd rather not just have this rubberstamped.

https://github.com/llvm/llvm-project/pull/87330