[llvm-config] Fix bug where `--libfiles` and `--names` would produce
incorrect output when LLVM is built with `LLVM_BUILD_LLVM_DYLIB`.
`llvm-config` previously produced output like this
```
$ llvm-config --libfiles
/usr/lib/liblibLLVM-4.0svn.so.so
$ llvm-config --libnames
liblibLLVM-4.0svn.so.so
```
The library prefix and shared library extension were added to
the library name twice which was wrong.
I wanted to write a test cases for this but it looks like **all**
`llvm-config` tests were disabled by r260386 so I'll leave this for
now.
Subscribers: llvm-commits, tstellarAMD
Reviewers: beanz, DiamondLovesYou, axw
Differential Revision: https://reviews.llvm.org/D27393
llvm-svn: 289488