Sync to upstream/release/536

[zeux]

• Fix bugs in type normalization of self-recursive union types
• Fix interaction between never types and operator type inference
• Fix type widening for functions that return a union of singleton string types
• string.match/string.find now return a more precise type if the match pattern is a string literal
• Type checker now supports passing strings to functions that use a string-like API even if the argument types aren't annotated.
• Add support for --!optimize to be able to control optimization level from source
• Compiler now replaces some calls to builtin functions with a constant if all arguments are known when using -O2
• Compiler now recognizes builtin calls for the purpose of cost modeling, which improves inlining/unrolling of functions with builtin calls
• lua_breakpoint can now place the breakpoint after the requested line if the requested line doesn't correspond to an executable location; it returns the line where the breakpoint is placed, or -1 on error.
• lua_Callbacks::useratom is now called lazily when the atom is requested via lua_tostringatom/lua_namecallatom, instead of eagerly during string construction.