Reinstate r185229, reverted in r185256, with a tweak: further ignore the
standard's rule that an extern "C" declaration conflicts with any entity in the
global scope with the same name. Now we only care if the global scope entity is
a variable declaration (and so might have the same mangled name as the extern
"C" declaration). This has been reported as a standard defect.

Original commit message:

PR7927, PR16247: Reimplement handling of matching extern "C" declarations
across scopes.

When we declare an extern "C" name that is not a redeclaration of an entity in
the same scope, check whether it redeclares some extern "C" entity from another
scope, and if not, check whether it conflicts with a (non-extern-"C") entity in
the translation unit.

When we declare a name in the translation unit that is not a redeclaration,
check whether it conflicts with any extern "C" entities (possibly from other
scopes).


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185281 91177308-0d34-0410-b5e6-96231b3b80d8
Teach -Wunsequenced that the side-effects of a function evaluation are sequenced
before the value computation of the result. In C, this is implied by there being
a sequence point after their evaluation, and in C++, it's implied by the
side-effects being sequenced before the expressions and statements in the
function body.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185282 91177308-0d34-0410-b5e6-96231b3b80d8