[LinkerScript] Implement linker script expression evaluation

[LinkerScript] Implement linker script expression evaluation

The expression evaluation is needed when interpreting linker scripts, in order
to calculate the value for new symbols or to determine a new position to load
sections in memory. This commit extends Expression nodes from the linker script
AST with evaluation functions, and also contains a unit test.

http://reviews.llvm.org/D8156

llvm-svn: 231707