[flang-commits] [flang] [flang] Add install target to install flang headers (PR #78151)

[Tom Eccles via flang-commits]

tblah wrote:

I wonder if there should be a single overall target covering flang-new, the runtime libraries, and the headers. But I am unsure what should call it. Just `install-flang` would match clang, but would collide with the target to only install the driver once the driver is renamed.

https://github.com/llvm/llvm-project/pull/78151