[ELF] - Support "INSERT AFTER" statement.

This implements INSERT AFTER in a following way:

During reading scripts it collects all insert statements.
After we done and read all files it inserts statements into script commands list.

With that:
* Rest of code does know nothing about INSERT.
* Approach is straightforward and have no visible limitations.
* It is also easy to support INSERT BEFORE (was seen in clang code once).
* Should work for PR35877 and similar cases.

Cons:
* It assumes we have "main" scripts that describes sections.

Differential revision: https://reviews.llvm.org/D43468

llvm-svn: 327003