if fast-completion is enabled. So they have higher chance to use the cached
completion instance.
If it's disabled, don't block, use an ephemeral instance so we can peform
multiple completions simultaneously.
To controls the lifetime of CompilerInstance within CompletionIntance.
- Prevent from using the same CompilerInstance from multiple completion
requests
- Separate the stacktrace between "fast" and "normal" completions
- Further code consolidation between various completion-like requests
- Introduce ide::CompletionInstance to manage CompilerInstance
- `CompletionInstance` vends the cached CompilerInstance when:
-- The compiler arguments (i.e. CompilerInvocation) has has not changed
-- The primary file is the same
-- The completion happens inside function bodies in both previous and
current completion
-- The interface hash of the primary file has not changed
- Otherwise, it vends a fresh CompilerInstance and cache it for the next
completion
rdar://problem/20787086
