[clang] Merge lifetimebound and GSL code paths for lifetime analysis (llvm#104906)

In the current lifetime analysis, we have two parallel code paths: one
for lifetimebound and another for GSL. These paths perform the same
logic, both determining whether to continue visiting subexpressions.

This PR merges the two paths into a single code path. As a result, we'll
reduce the overhead by eliminating a redundant visit to subexpressions.
The change is mostly NFC (No Functional Change). The only notable
difference is that when a subexpression is visited due to either
lifetimebound or GSL, we will prioritize the lifetimebound path. This
means the final diagnostic will be -Wdangling (rather than both
`-Wdangling` and `-Wdangling-gsl`)

This might cause a slight change in behavior if the -Wdangling
diagnostic is disabled, but I think this is not a major concern since
both diagnostics are enabled by default.

Fixes llvm#93386

Author: hokein

clang/docs/ReleaseNotes.rst

@@ -239,6 +239,8 @@ Improvements to Clang's diagnostics
 
 - Clang now diagnoses when the result of a [[nodiscard]] function is discarded after being cast in C. Fixes #GH104391.
 
+- Don't emit duplicated dangling diagnostics. (#GH93386).
+
 Improvements to Clang's time-trace
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