Add mangling for a call expression with semantically significant parentheses

[hubert-reinterpretcast]

See
#188 (comment)

[hubert-reinterpretcast]

Notes re: CWG 2946:

• Under the current wording of the ISO/IEC C++ Working Draft, the cp mangling is insufficient because (as reported in CWG 2946) the resolution of CWG 1321 does not apply to cases with parentheses.
• Additional cases that use the cl mangling may require different manglings after CWG 2946 is resolved (e.g., for friend declarations); however, the case for using the cp mangling is expected to remain.
• The implementation of cp mangling in Clang suffers from issues related to CWG 2946 because cases that mangle differently (between cl and cp) are considered to have equivalent types.

[rjmccall]

I feel like this merits a paragraph below explaining the rule exactly. Maybe something like this:

In C++, a call expression where the callee operand is an unqualified name uses argument-dependent lookup unless unqualified lookup finds certain kinds of declarations; see [class.lookup.argdep]p3. Because this rule does not apply when the name is parenthesized, it is sometimes necessary to distinguish parenthesized and unparenthesized calls in the mangling, despite the general rule that parentheses can be ignored. This is encoded using the choice of cl or cp for the call expression. The cp mangling is used only when the callee operand is a parenthesized unresolved name and would have used ADL if it were not parenthesized. In particular, cl is still used when unqualified lookup finds a declaration that would suppress the use of ADL, such as a class member.

[hubert-reinterpretcast]

I've added the paragraph. I indicated that the reference is to C++11 because the paragraph number for the rule was different before and has since changed. The wording around the block scope case was not present in C++03.

[rjmccall]

Thanks, LGTM. I'll leave this up for a week for further comments.