libclang output name is now libclang. This solves a name collision

when building with Visual Studio. `clang.dll' and `clang.exe' would
have the same `clang.ilk' and `clang.pdb'. On a serial build those
files would be overwritten as clang.exe/clang.dll are created. On a
parallel build there is a risk of both files being written at the same
time. On that case VS fails.

llvm-svn: 129239