[DebugInfo] [DWARFv5] Unique abbrevs for DIEs with different implicit_const values

Take DW_FORM_implicit_const attribute value into account when profiling
DIEAbbrevData.

Currently if we have two similar types with implicit_const attributes and
different values we end up with only one abbrev in .debug_abbrev section.
For example consider two structures: S1 with implicit_const attribute ATTR
and value VAL1 and S2 with implicit_const ATTR and value VAL2.
The .debug_abbrev section will contain only 1 related record:

[N] DW_TAG_structure_type       DW_CHILDREN_yes
        DW_AT_ATTR        DW_FORM_implicit_const  VAL1
        // ....

This is incorrect as struct S2 (with VAL2) will use abbrev record with VAL1.

With this patch we will have two different abbreviations here:

[N] DW_TAG_structure_type       DW_CHILDREN_yes
        DW_AT_ATTR        DW_FORM_implicit_const  VAL1
        // ....

[M] DW_TAG_structure_type       DW_CHILDREN_yes
        DW_AT_ATTR        DW_FORM_implicit_const  VAL2
        // ....

llvm-svn: 296691