[MC] Improve COFF associative section lookup

Handle the case when the symbol is private. Private symbols are not in
the COFF object file symbol table, so they aren't inserted into
SymbolMap. We can't look up the section of the symbol that way. Instead,
get the MCSection from the MCSymbol and map that to the object file
section.

Print a better error message when the symbol has no section, like when
the symbol is undefined.

Fixes PR38607

llvm-svn: 339942