[DebugInfo] Omit location list entries with empty ranges

Summary:
This fixes PR39710. In that case we emitted a location list looking like
this:

.Ldebug_loc0:
        .quad   .Lfunc_begin0-.Lfunc_begin0
        .quad   .Lfunc_begin0-.Lfunc_begin0
        .short  1                       # Loc expr size
        .byte   85                      # DW_OP_reg5
        .quad   .Lfunc_begin0-.Lfunc_begin0
        .quad   .Lfunc_end0-.Lfunc_begin0
        .short  1                       # Loc expr size
        .byte   85                      # super-register DW_OP_reg5
        .quad   0
        .quad   0

As seen, the first entry's beginning and ending addresses evalute to 0,
which meant that the entry inadvertently became an "end of list" entry,
resulting in the location list ending sooner than expected.

To fix this, omit all entries with empty ranges. Location list entries
with empty ranges do not have any effect, as specified by DWARF, so we
might as well drop them:

"A location list entry (but not a base address selection or end of list
 entry) whose beginning and ending addresses are equal has no effect
 because the size of the range covered by such an entry is zero."

Reviewers: davide, aprantl, dblaikie

Reviewed By: aprantl

Subscribers: javed.absar, JDevlieghere, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D55919

llvm-svn: 350698