Fixes various bugs around the keywords class, struct and union.

This switches to parsing record definitions only if we can clearly
identify them. We're specifically allowing common patterns for
visibility control through macros and attributes, but we cannot
currently fix all instances. This fixes all known bugs we have though.

Before:
static class A f() {
  return g();
} int x;

After:
static class A f() {
  return g();
}
int x;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172530 91177308-0d34-0410-b5e6-96231b3b80d8