IAS: Use the root macro instanciation for location

r224810 fixed the handling of macro debug locations in AsmParser. This patch
fixes the logic to actually do what was intended: it uses the first macro of
the macro stack instead of the last one. The updated testcase shows that the
current scheme doesn't work when macro instanciations are nested and multiple
files are used.

Reviewers: compnerd

Differential Revision: http://reviews.llvm.org/D10463

llvm-svn: 240705