Improve code completion by introducing patterns for the various C and

Improve code completion by introducing patterns for the various C and
C++ grammatical constructs that show up in top-level (namespace-level)
declarations, member declarations, template declarations, statements,
expressions, conditions, etc. For example, we now provide a pattern
for

  static_cast<type>(expr)

when we can have an expression, or

  using namespace identifier;

when we can have a using directive.

Also, improves the results of code completion at the beginning of a
top-level declaration. Previously, we would see value names (function
names, global variables, etc.); now we see types, namespace names,
etc., but no values.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@93134 91177308-0d34-0410-b5e6-96231b3b80d8