Introduce a new PresumedLoc class to represent the concept of a location

Introduce a new PresumedLoc class to represent the concept of a location
as reported to the user and as manipulated by #line.  This is what __FILE__,
__INCLUDE_LEVEL__, diagnostics and other things should follow (but not
dependency generation!).

This patch also includes several cleanups along the way:

- SourceLocation now has a dump method, and several other places
  that did similar things now use it.
- I cleaned up some code in AnalysisConsumer, but it should probably be
  simplified further now that NamedDecl is better.
- TextDiagnosticPrinter is now simplified and cleaned up a bit.

This patch is a prerequisite for #line, but does not actually provide
any #line functionality.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63098 91177308-0d34-0410-b5e6-96231b3b80d8