[libcxx] Remove ASan container overflow checks for SSO strings (#194208) Essentially a revert of #79536, although manual given how much the code has changed. Doing container overflow checks on SSO strings is fundamentally unsound due to how they interact with compiler optimizations within LLVM, which assume that the entire object is dereferencable when it is live. LLVM can sometimes insert speculative loads that will create false positives when they try and load a partially filled SSO buffer. This was previously prevented using inline asm barriers within std::string itself. However, this can also manifest in other ways, with the motivating example for the revert being GlobalOpt inserting a speculative load of a std::variant containing a SSO std::string, causing an ASan failure.
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