Enable Validating Header Dependencies

[envp]

Issue number of the reported bug or feature request: Internal ticket

Describe your changes We add the ability to track live dependencies of a header in the include graph through counts of:

• macro references
• variable references
• name re-declarations
• type references

provided by file uid A through file uid B. We call this the 'usage reference count' of the edge {A, B}.

Add new method:

• std::set<FileGraphEdge> IncludeGraph::liveDependencies(FileUID headerUID)
  • Return the set of headers that provide symbols, macros, or types referenced by the source in the file with UID headerUID
• bool IncludeGraph::decrementRefCount(FileGraphEdge edge)
  • For and edge in the include graph, decrement the usage reference count of this edge, returning a boolean indicating the success of the operation. Operation is considered failed if the edge doesn't exist or if the count is already 0.

Testing performed Added a test case t/031-validate-incldue-graph.t.cpp for the features proposed.

Test suite passes locally on:

• LLVM-8 / Darwin 18.6.0
• docker with LLVM-7 / ubuntu:18.04
• docker with LLVM-8 / ubuntu:18.10
• docker with LLVM-9 / ubuntu:19.04

[envp]

cc @dbeer1 @ruoso @azeemba @liangti

[envp]

@dbeer1 This is ready for review again.

[envp]

I've rebased the 'extra' commits into the intended parent.

[envp]

@azeemba This is good to merge now.