Debug info: (Bugfix) Make sure artificial functions like _GLOBAL__I_a

Debug info: (Bugfix) Make sure artificial functions like _GLOBAL__I_a
are not associated with any source lines.

Previously, if the Location of a Decl was empty, EmitFunctionStart would
just keep using CurLoc, which would sometimes be correct (e.g., thunks)
but in other cases would just point to a hilariously random location.

This patch fixes this by completely eliminating all uses of CurLoc from
EmitFunctionStart and rather have clients explicitly pass in a
SourceLocation for the function header and the function body.

rdar://problem/14985269

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205999 91177308-0d34-0410-b5e6-96231b3b80d8