llvm incorrect path, fix scan-build

`install_prefix` creates excessive nesting inside the library folder

Example:
```
$ llvm-config-3.5 --includedir
/usr/local/Cellar/llvm35/3.5.0/lib/llvm-3.5/include
$ llvm-config-3.5 --bindir
/usr/local/Cellar/llvm35/3.5.0/lib/llvm-3.5/bin
$  tree -d -L 3 /usr/local/Cellar/llvm35/3.5.0
/usr/local/Cellar/llvm35/3.5.0/
├── bin
├── lib
│   ├── llvm-3.5
│   │   ├── bin
│   │   ├── docs
│   │   ├── include
│   │   ├── lib
│   │   ├── libexec
│   │   ├── share
│   │   └── usr
│   └── python2.7
│       └── site-packages
└── share
    ├── clang-3.5
    │   └── tools
    └── man
        └── man1
```

bug Homebrew/homebrew#33777