[RISCV] Fix evaluation of %pcrel_lo

The following testcase

  function:
  .Lpcrel_label1:
        auipc   a0, %pcrel_hi(other_function)
        addi    a1, a0, %pcrel_lo(.Lpcrel_label1)
        .p2align        2          # Causes a new fragment to be emitted

        .type   other_function,@function
  other_function:
        ret

exposes an odd behaviour in which only the %pcrel_hi relocation is
evaluated but not the %pcrel_lo.

  $ llvm-mc -triple riscv64 -filetype obj t.s | llvm-objdump  -d -r -

  <stdin>:      file format ELF64-riscv

  Disassembly of section .text:
  0000000000000000 function:
         0:     17 05 00 00     auipc   a0, 0
         4:     93 05 05 00     mv      a1, a0
                0000000000000004:  R_RISCV_PCREL_LO12_I other_function+4

  0000000000000008 other_function:
         8:     67 80 00 00     ret

The reason seems to be that in RISCVAsmBackend::shouldForceRelocation
we only consider the fragment but in RISCVMCExpr::evaluatePCRelLo we
consider the section. This usually works but there are cases where
the section may still be the same but the fragment may be another
one. In that case we end forcing a %pcrel_lo relocation without any
%pcrel_hi.

This patch makes RISCVAsmBackend::shouldForceRelocation use the
section, if any, to determine if the relocation must be forced or
not.

Differential Revision: https://reviews.llvm.org/D60657