[PECOFF] Make a separate pass for /alternatename symbols.

/alternatename is a command line option to define a weak alias. You
can use it as /alternatename:foo=bar to define "foo" as a weak alias
for "bar".

Because it's a command line option, the weak alias mapping is in the
LinkingContext object, and not in a object file being read.

Previously, we looked up the mapping each time we read a new symbol
from a file, to check if there is a weak alias defined for the symbol.
That's not wrong, but had made function signature's a bit complicated --
we had to pass the mapping object to many functions. Now their
parameter lists are much cleaner.

This also has another (unrealized) benefit. parseFile() now read a
file and then add alias symbols to the file. In the first pass a
LinkingContext object is not used at all. That should make it easy
to read files from archive files speculatively, as the first pass
is free from side effect.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@209486 91177308-0d34-0410-b5e6-96231b3b80d8