[LinkerScript] Implement `MEMORY` command

As specified here:

* https://sourceware.org/binutils/docs/ld/MEMORY.html#MEMORY

There are two deviations from what is specified for GNU ld:

  1. Only integer constants and *not* constant expressions
     are allowed in `LENGTH` and `ORIGIN` initializations.

  2. The `I` and `L` attributes are *not* implemented.

With (1) there is currently no easy way to evaluate integer
only constant expressions.  This can be enhanced in the
future.

With (2) it isn't clear how these flags map to the `SHF_*`
flags or if they even make sense for an ELF linker.

Differential Revision: https://reviews.llvm.org/D28911

llvm-svn: 292875