[SR-8706] Fix up parseDependencyFile to not crash on invalid YAML

Previous ID	SR-8706
Radar	rdar://problem/25095498
Original Reporter	@belkadan
Type	Bug

Votes	0
Component/s	Compiler
Labels	Bug, Driver, StarterBug
Assignee	kushaj (JIRA)
Priority	Medium

md5: dfcb979cb9d3bef3f8ec210cda836a79

relates to:

• SR-8707 Write out swiftdeps files atomically

Issue Description:

LLVM's YAML parser returns null when it encounters a malformed node, but parseDependencyFile (in DependencyFile.cpp) isn't checking for this. We should fix this so that a malformed "swiftdeps" file doesn't result in crashes.

This applies to all uses of the LLVM YAML parser, really:

• OutputFileMap::parse in OutputFileMap.cpp

• populateOutOfDateMap in Driver.cpp

• YamlGroupInputParser in Serialization.cpp already seems to be doing this correctly.

• APIDiffItemStore and SILPassPipelinePlan::getPassPipelineFromFile don't really have to care about malformed input.

Tests for these APIs can go into the unittests/Driver/ folder.