[clang] [NFCI][analyzer] Simplify NodeBuilder constructors (PR #181875)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

I originally proposed the removal of `assert(hasNoSinksInFrontier());` in commit https://github.com/llvm/llvm-project/pull/181431/commits/782464859ee52fd29d4b57b06cf82e92b2dc4a60 on my previous PR https://github.com/llvm/llvm-project/pull/181431 -- but I moved it here because it was not connected to the rest of that PR.

The difference between that proposal and the first part of this PR (https://github.com/llvm/llvm-project/pull/181875/commits/29455b95eb3119ecd5cf25a641697ead18aad159) is that
- there I included defensive code to filter out the sink nodes from the source set and spoke about crash risk in `processBranch`;
- here I just remove the assertion and justify it with consistency and simplification (because the second part of this PR outright removes the `SrcSet` argument of the constructor of `BranchNodeBuilder`).

https://github.com/llvm/llvm-project/pull/181875