Get function start line number from DWARF info

DWARF info contains info about the line number at which a function starts (DW_AT_decl_line).

This patch creates a function to look up the start line number for a function, and returns it in
DILineInfo when looking up debug info for a particular address.

Patch by Simon Que!

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D27962

llvm-svn: 294231