syntax: Join consecutive string literals in format strings together

Emit a single rt::Piece per consecutive string literals. String literals
are split on {{ or }} escapes.

Saves a small amount of static storage and emitted code size.
Update LLVM to include NullCheckElimination pass

Fixes #11751
Rename to_str to to_string

Closes #15796.

[breaking-change]
Remove useless RefCells
Restructure test harness

We now build up a set of modules that reexport everything the test
framework needs, instead of turning off privacy.
Purge !resolve_unexported
Don't create reexport module if there are none
privacy: Add publically-reexported foreign item to exported item set

Close #15740
fix string in from_utf8_lossy_100_multibyte benchmark
Get rid of few warnings in tests
Clarify the std::vec::Vec docs regarding capacity
Test fixes from the rollup

Closes #15690 (Guide: improve error handling)
Closes #15729 (Guide: guessing game)
Closes #15751 (repair macro docs)
Closes #15766 (rustc: Print a smaller hash on -v)
Closes #15815 (Add unit test for rlibc)
Closes #15820 (Minor refactoring and features in rustc driver for embedders)
Closes #15822 (rustdoc: Add an --extern flag analagous to rustc's)
Closes #15824 (Document Deque trait and bitv.)
Closes #15832 (syntax: Join consecutive string literals in format strings together)
Closes #15837 (Update LLVM to include NullCheckElimination pass)
Closes #15841 (Rename to_str to to_string)
Closes #15847 (Purge #[!resolve_unexported] from the compiler)
Closes #15848 (privacy: Add publically-reexported foreign item to exported item set)
Closes #15849 (fix string in from_utf8_lossy_100_multibyte benchmark)
Closes #15850 (Get rid of few warnings in tests)
Closes #15852 (Clarify the std::vec::Vec::with_capacity docs)
Lexer; subtly wrong; no makefile
Add raw string literal ambiguity document
First pass at line comment correctness
Refine the tooling, handle comments
Byte/raw binary literal fixes
ignore-lexer-test to broken files and remove some tray hyphens

I blame @ChrisMorgan for the hyphens.
rustc: Append platform exe suffix to output files

Closes #15828
auto merge of #15864 : alexcrichton/rust/rollup, r=alexcrichton