diagtool: refactor TreeView to resemble C++

Replace lots of old-school parameter passing with neat class members.
No attempt made yet to modernize loops, but it's a start.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211327 91177308-0d34-0410-b5e6-96231b3b80d8
Avoid invalidating successfully loaded module files

Successfully loaded module files may be referenced in other
ModuleManagers, so don't invalidate them. Two related things are fixed:

1) I thought the last module in the manager was always the one that
failed, but it isn't.  So check explicitly against the list of
vetted modules from ReadASTCore.

2) We now keep the file descriptor of pcm file open, which avoids the
possibility of having two different pcms for the same module loaded when
building in parallel with headers being modified during a build.

<rdar://problem/16835846>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211330 91177308-0d34-0410-b5e6-96231b3b80d8