[MC] Delete unnecessary diagnostic: "No relocation available to represent this relati...

[MC] Delete unnecessary diagnostic: "No relocation available to represent this relative expression"

Replace

- error: No relocation available to represent this relative expression

with

+ error: symbol 'undef' can not be undefined in a subtraction expression

or

+ error: Cannot represent a difference across sections

Keep !IsPcRel as an assertion after the two diagnostic checks are done.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369239 91177308-0d34-0410-b5e6-96231b3b80d8