Use -fms-compatibility to trigger lookup into dep. bases

Update the docs for -fms-extensions and -fms-compatibility to try to
clarify the difference between the two.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191095 91177308-0d34-0410-b5e6-96231b3b80d8
Clarify that we silently ignore some MS extensions like IDL attributes

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191096 91177308-0d34-0410-b5e6-96231b3b80d8
Fix ReST links I just added to the manual

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191097 91177308-0d34-0410-b5e6-96231b3b80d8