Extend the code-completion caching infrastructure to include global

Extend the code-completion caching infrastructure to include global
declarations (in addition to macros). Each kind of declaration maps to
a certain set of completion contexts, and the ASTUnit completion logic
introduces the completion strings for those declarations if the actual
code-completion occurs in one of the contexts where it matters.

There are a few new code-completion-context kinds. Without these,
certain completions (e.g., after "using namespace") would need to
suppress all global completions, which would be unfortunate.

Note that we don't get the priorities right for global completions,
because we don't have enough type information. We'll need a way to
compare types in an ASTContext-agnostic way before this can be
implemented.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@111093 91177308-0d34-0410-b5e6-96231b3b80d8