[clangd] Do not end inactiveRegions range at position 0 of line

This carries over the fix previously made for semantic highlighting
https://reviews.llvm.org/D92148, to the new inactiveRegions
protocol as well.

In addition, the directives at the beginning and end of an
inactive region are now excluded from the region.

Fixes https://github.com/clangd/clangd/issues/1631
Fixes https://github.com/clangd/clangd/issues/773

Differential Revision: https://reviews.llvm.org/D151190