MC: correct the emission of weak aliases in COFF

The weak alias should have the characteristics set to
`IMAGE_EXTERN_WEAK_SEARCH_ALIAS` to indicate that the weak external here
is a symbol alias and that the symbol is aliased to a locally defined
symbol.  We were previously setting the characteristics to
`IMAGE_EXTERN_WEAK_SEARCH_LIBRARY` which indicates that the symbol
should be looked for in the libraries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364370 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed