This patch implements location counter support.
It also separates assign addresses for sections to assignAddressesScript() if it scipt exists.
Main testcase is test/ELF/linkerscript-locationcounter.s, It contains some work with location counter. It is basic now.
Implemented location counter assignment and '+' operations.
I'd make this a enum class and rename members as shown below.
LocNodeType -> Command
LocSet -> Expr
LocOutSet -> Section