Recommitted r267132 "[ELF] - implemented ternary operator for linkerscript expressions"

With fix: removed redundant Dot parameter.

Original commit message:
[ELF] - implemented ternary operator for linkerscript expressions

Patch implements ternary operator for linkerscript expressions.
Like:

SECTIONS {
 . = 0x1 ? 0x2 : 0x3;
...
}

Differential revision: http://reviews.llvm.org/D19332

llvm-svn: 267140