LinkerScript: Add parsing of the MEMORY command

This patch implements parsing of the GNU ld MEMORY command [1].
The command and the memory block definitions are parsed as
specified (including the slightly strange "o" and "l" keywords).
Evaluation will be added at a later point in time.

[1] https://sourceware.org/binutils/docs-2.25/ld/MEMORY.html

llvm-svn: 231928