[ELF] Check COMMON symbols for PROVIDE and don't redefine COMMON symbols edata/end/etext

In GNU ld, the definition precedence is: regular symbol assignment > relocatable object definition > `PROVIDE` symbol assignment.

GNU ld's internal linker scripts define the non-reserved (by C and C++)
edata/end/etext with `PROVIDE` so the relocatable object definition takes
precedence. This makes sense because `int end;` is valid.

We currently redefine such symbols if they are COMMON, but not if they are
regular definitions, so `int end;` with -fcommon is essentially a UB in ld.lld.
Fix this (also improve consistency and match GNU ld) by using the
`isDefined` code path for `isCommon`. In GNU ld, reserved identifiers like
`__ehdr_start` do not use `PROVIDE`, while we treat them all as `PROVIDE`, this
seems fine.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D120389