[LLD][ELF] Implement symbol assignment within OVERLAY sections in linker script handling of LLD

[CompilerWang]

Consider the following incomplete linker script snippet:

SECTIONS {
  OVERLAY 0x1000 : AT ( 0x4000 ) {
    .out.big { 
      *(.big1)
      . = ALIGN(0x100);
      *(.big2)
    }
    .out.small { *(.small) }
  }
}

LLD fails with ( expected, but got = while processing . = ALIGN(0x100);
Ideally, we would like to match the BFD linker behavior for symbol assignment in this case.

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: None (CompilerWang)

Consider the following incomplete linker script snippet: ``` SECTIONS { OVERLAY 0x1000 : AT ( 0x4000 ) { .out.big { *(.big1) . = ALIGN(0x100); *(.big2) } .out.small { *(.small) } } } ```

LLD fails with ( expected, but got = while processing . = ALIGN(0x100);
Ideally, we would like to match the BFD linker behavior for symbol assignment in this case.