Clang crash targeting ARM or Thumb when assembling a prel31 relocation variant

Summary:
In the assembler or inline assembler,
attempting to use an invalid fixup type
gives a crash with a segmentation fault.

__attribute__((naked))
void foo(void) {
 __asm__("mov r9, :lower16:bar(prel31)");
}

This should give a proper error message when building for ARM or Thumb.
This brings it in line with AARCH64.

This fixes all 8 instances of llvm_unreachable("Unsupported Modifier");
in ARM/MCTargetDesc/ARMELFObjectWriter.cpp.
A test is provided for each instance.

Reviewers: llvm-commits, MarkMurrayARM

Reviewed By: MarkMurrayARM

Subscribers: kristof.beyls, hiraditya, danielkiss

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79782

Change-Id: I6971ba37f129cc453568fe71514ccb2ac9d16831