Various crash reporting tools have a problem with the dwarf generated for

Various crash reporting tools have a problem with the dwarf generated for
assembly source when it generates the TAG_subprogram dwarf debug info for
the labels that have nothing between them as in this bit of assembly source:

% cat ZeroLength.s
_func1:
_func2:
 nop

One solution would be to not emit the subsequent labels with the same address
and use the next label with a different address or the end of the section for
the AT_high_pc value of the TAG_subprogram.

Turns out in llvm-mc it is not possible in all cases to determine of two
symbols have the same value at the point we put out the TAG_subprogram dwarf
debug info.

So we will have llvm-mc instead of putting out TAG_subprogram's put out
DW_TAG_label's.  And the DW_TAG_label does not have a AT_high_pc value which
avoids the problem.

This commit is only the functional change to make the diffs clear as to what is
really being changed.  The next commit will be to clean up the names of such
things like MCGenDwarfSubprogramEntry to something like MCGenDwarfLabelEntry.

rdar://10666925

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147860 91177308-0d34-0410-b5e6-96231b3b80d8