[clang][Sema] Two-pass noreturn inference via CFG for always-throwing functions

[snarang181]

This patch adds a new AnalysisBasedWarnings pass that runs right before
IssueWarnings:

1. Pass 1 seeds trivial always-throwing functions.
2. Pass 2 performs a fixed-point CFG-based analysis (CheckFallThrough) to propagate noreturn inference to any caller whose all paths end in calls to already-inferred noreturn functions.

To support templates, we also walk each FunctionTemplateDecl’s specializations so that instantiations (e.g. ensureZeroTemplate) get analyzed.