[clangd] Canonicalize compile flags before applying edits

Pushes input for the compile action to the end while separating with a
`--` before applying other manglings. This ensures edits that effect only the
arguments that come after them works, like changing parse language via -x.

Fixes https://github.com/clangd/clangd/issues/555.

Differential Revision: https://reviews.llvm.org/D106527