[ELF] - Also use DW_AT_linkage_name when gathering information about variables for error messages.

Currently, when LLD do a lookup for variables location, it uses DW_AT_name attribute.
That is not always enough.

Imagine code:

namespace A {
  int bar = 0;
}

namespace Z {
  int bar = 1;
}

int hoho;
In this case there are 3 variables and their debug attributes are following:

A::bar has: DW_AT_name [DW_FORM_string] ("bar") DW_AT_linkage_name [DW_FORM_strp] ( .debug_str[0x00000006] = "_ZN1A3barE")
Z::bar has: DW_AT_name [DW_FORM_string] ("bar") DW_AT_linkage_name [DW_FORM_strp] ( .debug_str[0x0000003f] = "_ZN1Z3barE")
hoho has: DW_AT_name [DW_FORM_strp] ( .debug_str[0x0000004a] = "hoho") and has NO DW_AT_linkage_name attribute. Because it would be
the same as DW_AT_name and DWARF producers avoids emiting excessive data.

Hence LLD should also use DW_AT_linkage_name when it is available.
(currently, LLD fails to report location correctly because thinks that A::bar and Z::bar are the same things)

Differential revision: https://reviews.llvm.org/D47373

llvm-svn: 333880