(1) Add llvm_expect(), an asserting macro that can be evaluated as a constexpr
expression as well as a runtime assert or compiler hint in release builds. This
technique can be used to construct functions that are both unevaluated and
compiled depending on usage.
(2) Update StringRef using llvm_expect() to preserve runtime assertions while
extending the same checks to static asserts in C++11 builds that support the
feature.
(3) Introduce ConstStringRef, a strong subclass of StringRef that references
compile-time constant strings. It's convertible to, but not from, ordinary
StringRef and thus can be used to add compile-time safety to various interfaces
in LLVM and clang that only accept fixed inputs such as diagnostic format
strings that tend to get misused.
llvm-svn: 200187
042f41b047