This is a large rewrite of how Dwarf info for inlined functions is handled.

The DwarfWriter expects DbgScopes and DIEs to behave themselves according to
DwarfWriter's rules. However, inlined functions violate these rules. There are
two different types of DIEs associated with an inlined function: an abstract
instance, which has information about the original source code for the function
being inlined; and concrete instances, which are created for each place the
function was inlined and point back to the abstract instance.

This patch tries to stay true to this schema. It bypasses how regular DbgScopes
and DIEs are created and used when necessary. It provides special handling for
DIEs of abstract and concrete instances.

This doesn't take care of all of the problems with debug info for inlined
functions, but it's a step in the right direction. For one thing, llvm-gcc
generates wrong IR (it's missing some llvm.dbg intrinsics at the point where the
function's inlined) for this example:

#include <stdio.h>
static __inline__ __attribute__((always_inline))  int bar(int x) { return 4; }
void foo() {
  long long b = 1;
  int Y = bar(4);
  printf("%d\n", Y);
}

while clang generates correct IR.

llvm-svn: 71410