Allow for vendor prefixes in a list test

Summary:
Preprocessor/init.c contains a line that explicitly checks for the
string

__VERSION__ "Clang{{.*}}

It's valid to have a toolchain configured to emit a vendor prefix
before the word Clang. e.g.

__VERSION__ "Vendor Clang{{.*}}

Subscribers: fedor.sergeev, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64772

llvm-svn: 366159