[ELF] Implement parsing `-l` prefixed items in the `GROUP` linker script command.
There are two forms of `-l` prefixed expression:

* -l<libname>
* -l:<filename>

In the first case a linker should construct a full library name
`lib + libname + .[so|a]` and search this library as usual. In the second case
a linker should use the `<filename>` as is and search this file through library
search directories.

The patch reviewed by Shankar Easwaran.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@213077 91177308-0d34-0410-b5e6-96231b3b80d8