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[LLD] Search archives for symbol defs to override COMMON symbols.

Authored by sfertile on Dec 7 2020, 6:28 AM.

Description

[LLD] Search archives for symbol defs to override COMMON symbols.

This patch changes the archive handling to enable the semantics needed
for legacy FORTRAN common blocks and block data. When we have a COMMON
definition of a symbol and are including an archive, LLD will now
search the members for global/weak defintions to override the COMMON
symbol. The previous LLD behavior (where a member would only be included
if it satisifed some other needed symbol definition) can be re-enabled with the
option '-no-fortran-common'.

Differential Revision: https://reviews.llvm.org/D86142