[clang-tools-extra] [clangd] Introduce --skip-preamble-build command line option (PR #189284)

[Aleksandr Platonov via cfe-commits]

ArcsinX wrote:

I've added test (based on tests from https://github.com/llvm/llvm-project/pull/187432 ) and now this is ready for review.
In this test we check that when we skip preamble build:
- no errors in scenario described here https://github.com/llvm/llvm-project/issues/181770
- code completion works (for structure defined in the module and in the header)
- preamble size is zero

https://github.com/llvm/llvm-project/pull/189284