Follow LLVM style guidelines for use of data structures

[mattmccutchen-cci]

From review of a PR to Microsoft:

I noticed that clang/include/clang/3C/ProgramInfo.h uses std::map and clang/lib/3C.cpp uses
std::vector. Llvm recommends that: "llvm::DenseMap should almost always be used instead of std::map or
std::unordered_map, and llvm::SmallVector should usually be used instead of std::vector" .

Here are the LLVM guideline and the Programmer's Manual section on data structures.

[sulekhark]

Thank you! Please also consider using llvm::DenseSet or llvm::SmallSet instead of std::set.