[clang-include-fixer] Fix incorrect ranking because of dangling references

SymbolAndSignals stores SymbolInfo which stores two std::strings. Then
the values are stored in a llvm::DenseMap<llvm::StringRef, double>. When
the sorting is happening, SymbolAndSignals are swapped and thus because
of small string optimization some strings may become invalid. This
results in incorrect ranking.

This was detected when running new std::sort algorithm against llvm
toolchain. This could have been prevented with running llvm::sort and
EXPENSIVE_CHECKS. Unfortunately, no sanitizer yelled.

I don't have commit rights, kutdanila@yandex.ru Danila Kutenin

Reviewed By: bkramer

Differential Revision: https://reviews.llvm.org/D116037