[Debuginfo][NFC] Avoid double calling of DWARFDie::find(DW_AT_name).

Summary:
Current implementation of DWARFDie::getName(DINameKind Kind) could
lead to double call to DWARFDie::find(DW_AT_name) in following
scenario:

getName(LinkageName);
getName(ShortName);

getName(LinkageName) calls find(DW_AT_name) if linkage name is not
found. Then, it is called again in getName(ShortName). This patch
alows to request LinkageName and ShortName separately
to avoid extra call to find(DW_AT_name).

It helps D74169 to parse clang debuginfo faster(~1%).

Reviewers: clayborg, dblaikie

Differential Revision: https://reviews.llvm.org/D79173