Use class to pass information about executable name

Use class to pass information about executable name

Information about clang executable name components, such as target and
driver mode, was passes in std::pair. With this change it is passed in
a special structure. It improves readability and makes access to this
information more convenient.

NFC.

Differential Revision: https://reviews.llvm.org/D36057

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311981 91177308-0d34-0410-b5e6-96231b3b80d8