[clangd] Rewrite TweakTesting helpers to avoid reparsing the same code. NFC

[clangd] Rewrite TweakTesting helpers to avoid reparsing the same code. NFC

Previously the EXPECT_AVAILABLE macros would rebuild the code at each marked
point, by expanding the cases textually.
There were often lots, and it's nice to have lots!

This reduces total unittest time by ~10% on my machine.
I did have to sacrifice a little apply() coverage in AddUsingTests (was calling
expandCases directly, which was otherwise unused), but we have
EXPECT_AVAILABLE tests covering that, I don't think there's real risk here.

Differential Revision: https://reviews.llvm.org/D125109