Use an iterative method instead of recursion for printing macro backtraces.

Store the locations for a macro expansion in a vector, then iterate over them
instead of using recursion.  This simplifies the logic around the backtrace
limit and gives easier access to the source locations.  No functionality change.

Patch by Zhengkai Wu.

Differential Revision: http://reviews.llvm.org/D11542

llvm-svn: 243477