Test fixes from the rollup

Closes #14482 (std: Bring back half of Add on String)
Closes #15026 (librustc: Remove the fallback to `int` from typechecking.)
Closes #15119 (Add more description to c_str::unwrap().)
Closes #15120 (Add tests for #12470 and #14285)
Closes #15122 (Remove the cheat sheet.)
Closes #15126 (rustc: Always include the morestack library)
Closes #15127 (Improve ambiguous pronoun.)
Closes #15130 (Fix #15129)
Closes #15131 (Add the Guide, add warning to tutorial.)
Closes #15134 (Xfailed tests for hygiene, etc.)
Closes #15135 (core: Add stability attributes to Clone)
Closes #15136 (Some minor improvements to core::bool)
Closes #15137 (std: Add stability attributes to primitive numeric modules)
Closes #15141 (Fix grammar in tutorial)
Closes #15143 (Remove few FIXMEs)
Closes #15145 (Avoid unnecessary temporary on assignments)
Closes #15147 (Small improvements for metaprogramming)
Closes #15153 (librustc: Check function argument patterns for legality of by-move)
Closes #15154 (test: Add a test for regions, traits, and variance.)
Closes #15159 (rustc: Don't register syntax crates twice)
Closes #13816 (Stabilize version output for rustc and rustdoc)