[clang] [clang] Introduce copy-on-write `CompilerInvocation` (PR #65412)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

Made the entirety of `ValBase` ref-counted in https://github.com/llvm/llvm-project/pull/65647, which also resolves some other comments here.

https://github.com/llvm/llvm-project/pull/65412