[FastISel] Some long overdue spring cleaning of FastISel.

Things got a little bit messy over the years and it is time for a little bit
spring cleaning.

This first commit is focused on the FastISel base class itself. It doxyfies all
comments, C++11fies the code where it makes sense, renames internal methods to
adhere to the coding standard, and clang-formats the files.

Reviewed by Eric

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217060 91177308-0d34-0410-b5e6-96231b3b80d8