[Lex] Remove DirectoryLookup.UserSpecified, which is unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173409 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] The -iwithprefix option belongs in the After category, according to
GCC docs.
 - Found by inspection.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173410 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] Drop the isUserSupplied argument to InitHeaderSearch, it is unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173411 91177308-0d34-0410-b5e6-96231b3b80d8
[Frontend] Remove another IsUserSpecified member variable that is now unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173412 91177308-0d34-0410-b5e6-96231b3b80d8