llvm-symbolizer: Handle function definitions nested within other functions

LLVM always puts function definition DIEs at the top level, but under
some circumstances GCC does not (at least in this case with member
functions of a function-local type).

To ensure that doesn't appear as though the local type's member function
is unduly inlined within the outer function - ensure the inline
discovery DIE parent walk stops at the first DW_TAG_subprogram.

llvm-svn: 331291