[llvm] [Transforms] Allow non-regex Source in SymbolRewriter in case of using ExplicitRewriteDescriptor (PR #154319)

[Dmitry Vasilyev via llvm-commits]

slydiman wrote:

> Needs tests

I would be happy to add tests.
But I don't see any usage of SymbolRewriter in public LLVM tools.
Currently SymbolRewriter has no tests at all. 
So first it is necessary to create some environment to test SymbolRewriter with some source data.
This patch is safe and very simple (the regex checking has been moved after target usage). 
I hope this patch can be applied without tests.

https://github.com/llvm/llvm-project/pull/154319