[clang] WIP [clang][ssaf] Add TUSummaryBuilder and testing features (PR #180779)

[Balázs Benics via cfe-commits]

steakhal wrote:

I had to force push because some of the commits didn't make much sense after the discussion.
These commits were dropped:
 - `Add EntityLinkage stub`
 - `Add operator<< for debugging to SummaryName and EntityId`

https://github.com/llvm/llvm-project/pull/180779