DWARFv5: Handle location list for split dwarf.

DWARFv5: Handle location list for split dwarf.

GDB throws the error '<error reading variable: dwarf2_find_location_
expression: Corrupted DWARF expression.>' while printing the variable
value with executable file compiled with -gdwarf-5 and -gdwarf-split
flags. This is because DW_LLE_start* or DW_LLE_offset_pair with
DW_LLE_base_addressx are being emitted in DWARFv5 location list instead of
DW_LLE_GNU*. This patch fixes this error.

Tested by running the testsuite before and after the patch and there is no
increase in the number of test cases that fails. Tested with both -gdwarf-4
and -gdwarf-5 flags. Also tested -gslit-dwarf along with -gdwarf-4 as well as
-gdwarf-5 flags. Used clang version 10.0.0. This is the test case used-

void bar(int arr[], int l, int m, int r) {
    int i, j, k, n1= m - l + 1, n2= r - m, L[n1], R[n2];
    for (i = 0; i < n1; i++)
        L[i] = arr[l + i];
    for (j = 0; j < n2; j++)
        R[j] = arr[m + 1+ j];
}

int main()
{
    int arr[] = {12, 11};
    bar(arr,0,1,2);
    return 0;
}

clang -gdwarf-5 -gsplit-dwarf test.c -o test.out

gdb test.out
gdb> start
gdb> step
gdb> step
gdb> step
gdb> step
gdb> p L[0]
dwarf2_find_location_expression: Corrupted DWARF expression.

gdb/ChangeLog:
2020-04-07  Nitika Achra  <Nitika.Achra@amd.com>

	* dwarf2/loc.c (enum debug_loc_kind): Add a new kind DEBUG_LOC_OFFSET_PAIR.
	(dwarf2_find_location_expression): Call the function decode_debug_loclists_
	addresses if DWARF version is 5 or more. DW_LLE_start* or DW_LLE_offset_pair
	with DW_LLE_base_addressx are being emitted in DWARFv5 instead of DW_LLE_GNU*.
	Add applicable base address if the entry is DW_LLE_offset_pair from DWO.
	(decode_debug_loclists_addresses): Return DEBUG_LOC_OFFSET_PAIR instead of
	DEBUG_LOC_START_END in case of DW_LLE_offset_pair.