[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