section relative expressions

It looks like llvm-mc isn't treating section relative expressions correctly.

In this example:
.text
mov (.rodata+0), %rsi
.section .rodata
.byte 0xff

llvm-mc will create an undefined symbol for the expression, ".rodata+0" and a relocation will be generated against the undefined symbol not the section.

Should be fixed in r219835. Thanks!

Cheers,
Rafael