[clang-tidy] Add a little checker for Twine locals in LLVM.

Those often cause use after free bugs and should be generally avoided.
Technically it is safe to have a Twine with >=2 components in a variable
but I don't think it is a good pattern to follow. The almost trivial checker
comes with elaborated fix-it hints that turn the Twine into a std::string
if necessary and otherwise fall back to the original type if the Twine
is created from a single value.

llvm-svn: 212535