[clangd] Code completion for declaration of class method (#165916)

Code completion previously could not tell apart the declaration of
a method from a call to it, and provided call-like behaviour even
in declaration contexts. This included things like not offering 
completion for private methods, and inserting placeholders for
the parameters as though the user was going to fill in arguments.

This patch adds support to Parser and SemaCodeComplete to
detect and provide dedicated behaviour for declaration contexts.
In these contexts, the flag CodeCompletionResult::DeclaringEntity
is set, and createCodeCompletionString() is adjusted to handle this
flag, e.g. by inserting parameter declarations as text chunks rather
than placeholder chunks.

The DeclaringEntity flag is also available for consumers of
SemaCodeComplete, such as clangd, to customize their behaviour.

In addition, the patch tweaks the conditions under which the
existing CodeCompletionResult::FunctionCanBeCall flag is set to
be more accurate, excluding declaration contexts and cases where
the address of the function is likely being taken.

Fixes clangd/clangd#753
Fixes clangd/clangd#880
Fixes clangd/clangd#1752