Be more careful in applying pragma weak. Fixes pr14974.

GCC applies a pragma weak to a decl if it matches the mangled name. We used
to apply if it matched the plain name.

This patch is a compromise: we apply the pragma only if it matches the name
and the decl has C language linkage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176110 91177308-0d34-0410-b5e6-96231b3b80d8
Use the most recent decl in getExplicitVisibility.

Now that implicitly hidden template arguments can make an instantiation hidden,
it is important to look at more than just the canonical decl of the argument
in order to see if an attribute is available in a more recent decl.

This has the disadvantage of exposing when getExplicitVisibility is called,
but lets us handle cases like

template <typename T>
struct __attribute__((visibility("default"))) barT {
  static void zed() {}
};
class foo;
class __attribute__((visibility("default"))) foo;
template struct barT<foo>;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176112 91177308-0d34-0410-b5e6-96231b3b80d8
Warn on dropping the return value from a warn_unused_result function, even in
macros.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176114 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Use 'MemRegion::printPretty()' instead of assuming the region is a VarRegion.

Fixes PR15358 and <rdar://problem/13295437>.

Along the way, shorten path diagnostics that say "Variable 'x'" to just
be "'x'".  By the context, it is obvious that we have a variable,
and so this just consumes text space.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176115 91177308-0d34-0410-b5e6-96231b3b80d8