[ELF] Combine foo@v1 and foo with the same versionId if both are defined

Due to an assembler design flaw (IMO), `.symver foo,foo@v1` produces two symbols `foo` and `foo@v1` if `foo` is defined.

* `v1 {};` produces both `foo` and `foo@v1`, but GNU ld only produces `foo@v1`
* `v1 { foo; };` produces both `foo@@v1` and `foo@v1`, but GNU ld only produces `foo@v1`
* `v2 { foo; };` produces both `foo@@v2` and `foo@v1`, matching GNU ld. (Tested by symver.s)

This patch implements the GNU ld behavior by reusing the symbol redirection mechanism
in D92259. The new test symver-non-default.s checks the first two cases.

Without the patch, the second case will produce `foo@v1` and `foo@@v1` which
looks weird and makes foo unnecessarily default versioned.

Note: `.symver foo,foo@v1,remove` exists but the unfortunate `foo` will not go
away anytime soon.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D107235