[clangd] GlobalCompilationDatabase interface changes

Summary:
- GlobalCompilationDatabase now returns a single command (that's all we use)
- fallback flags are now part of the GlobalCompilationDatabase.
  There's a default implementation that they can optionally customize.
- this allows us to avoid invoking the fallback logic on two separate codepaths
- race on extra flags fixed by locking the mutex
- made GCD const-correct (DBGCD does have mutating methods)

Reviewers: hokein

Subscribers: klimek, cfe-commits, ilya-biryukov

Differential Revision: https://reviews.llvm.org/D40733

llvm-svn: 319647