[Lexer] Don't merge macro args from different macro files
The lexer sets the end location of macro arguments incorrectly *if*,
while merging consecutive args to fit into a single SLocEntry, it finds
args which come from different macro files.
Fix the issue by using separate SLocEntries in this situation.
This fixes a code coverage crasher (rdar://problem/26181005). Because
the lexer reported end locations for certain macro args incorrectly, we
would generate bogus coverage mappings with negative line offsets.
Differential Revision: http://reviews.llvm.org/D20401