Add txt file extension to VS solution's README

This matches other README.txt files in LLVM and makes things more obvious on
Windows where it's likely to be read. CRLFs are retained for the same reason.

Also fix Visual Studio product name.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@195420 91177308-0d34-0410-b5e6-96231b3b80d8
Tidy up the no-external-assembler diag

Diags aren't usually in the first person, and 'windows' isn't the correct
product spelling to use in prose. Sidestep issues completely by making this
error message platform-neutral.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@195422 91177308-0d34-0410-b5e6-96231b3b80d8