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
$  ls /usr/local/Cellar/llvm35/3.5.0/lib/llvm-3.5/
bin     docs    include lib     libexec share   usr
```

bug Homebrew/homebrew#33777