[analyzer][PlistMacroExpansion] Part 1.: New expand-macros flag

This is the first part of the implementation of the inclusion of macro
expansions into the plist output. It adds a new flag that adds a new
"macro_expansions" entry to each report that has PathDiagnosticPieces that were
expanded from a macro. While there's an entry for each macro expansion, both
the name of the macro and what it expands to is missing, and will be implemented
in followup patches.

Differential Revision: https://reviews.llvm.org/D52742

llvm-svn: 345724