[asan] Enable StackSafetyAnalysis by default

StackSafetyAnalysis determines whether stack-allocated variables are
guaranteed to be safe from memory access bugs and enables the removal of
certain unneeded instrumentations.
(hwasan enables StackSafetyAnalysis in https://reviews.llvm.org/D108381)

In a release build of clang, text sections are 9% smaller.

Test updates:

* asan-stack-safety.ll: test the -asan-use-stack-safety=1 default
* lifetime-uar-uas.ll: switch to an indexed store to prevent
  StackSafetyAnalysis from optimizing out instrumentation for %c
* alloca_vla_interact.cpp: add a load to prevent StackSafetyAnalysis
  from optimizing out `__asan_alloca_poison` for the VLA `array`
* scariness_score_test.cpp: add -asan-use-stack-safety=0 to make a load
  of a `__asan_poison_memory_region`-poisoned local variable fail as
  intended.
* other .ll tests: add -asan-use-stack-safety=0

Reviewed By: kstoimenov

Pull Request: https://github.com/llvm/llvm-project/pull/77210