switch SourceManager from using an std::map and std::list of

switch SourceManager from using an std::map and std::list of
ContentCache objects to using a densemap and list, and allocating
the ContentCache objects from a bump pointer.  This does not speed
up or slow down things substantially, but gives us control over
their alignment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63628 91177308-0d34-0410-b5e6-96231b3b80d8