[lit] Optimize ShLexer string construction (#199641)
`lex_arg_quoted` and `lex_arg_slow` methods in the `lit`'s `ShLexer`
class used `+=` for string concatenation, which was inefficient. This change
replaces that pattern by collecting string fragments in a list and using
`"".join()` at the end.
**Baseline:**
| Stage | Metric (Runtime / Peak RSS) |
| :--- | :--- |
| **CodeGen-X86** | 99.310s / 51872 kB |
| **llvm-transforms** | 51.457s / 66928 kB |
**Changes made here:**
| Stage | Metric (Runtime / Peak RSS) |
| :--- | :--- |
| **CodeGen-X86** | 94.964s / 51868 kB |
| **llvm-transforms** | 50.890s / 66328 kB |
This PR is part of the "GSoC 2026: Improving lit" project.
Signed-off-by: Prasoon Kumar <prasoonkumar054@gmail.com>
