Simplify lifetime management of MacroInfo objects in Preprocessor by having...

Simplify lifetime management of MacroInfo objects in Preprocessor by having the Preprocessor maintain them in a linked
list of allocated MacroInfos.  This requires only 1 extra pointer per MacroInfo object, and allows us to blow them
away in one place.  This fixes an elusive memory leak with MacroInfos (whose exact location I couldn't still figure
out despite substantial digging).

Fixes <rdar://problem/8361834>.

llvm-svn: 116842