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PR40642: Fix determination of whether the final statement of a statement

Description

PR40642: Fix determination of whether the final statement of a statement
expression is a discarded-value expression.

Summary:
We used to get this wrong in three ways:

  1. During parsing, an expression-statement followed by the }) ending a statement expression was always treated as producing the value of the statement expression. That's wrong for ({ if (1) expr; })
  2. During template instantiation, various kinds of statement (most statements not appearing directly in a compound-statement) were not treated as discarded-value expressions, resulting in missing volatile loads (etc).
  3. In all contexts, an expression-statement with attributes was not treated as producing the value of the statement expression, eg ({ [[attr]] expr; }).

Also fix incorrect enforcement of OpenMP rule that directives can "only
be placed in the program at a position where ignoring or deleting the
directive would result in a program with correct syntax". In particular,
a label (be it goto, case, or default) should not affect whether
directives are permitted.

Reviewers: aaron.ballman, rjmccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57984