[clang] Fix format specifiers fixits for nested macros

ExpansionLoc was previously calculated incorrectly in the case of 
nested macros expansions. In this diff we build the stack of expansions 
where the last one is the actual expansion which should be used 
for grouping together the edits. 
The definition of MacroArgUse is adjusted accordingly.

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D34268

llvm-svn: 305845