Change scopes to maintain decls, not identifiers.

Detect and emit errors when names are redefined in the same scope, e.g.
test/Parser/argument_redef.c, which now emits:

argument_redef.c:4:22: error: redefinition of 'A'
int foo(int A) { int A; }
                     ^
argument_redef.c:4:13: error: previous definition is here
int foo(int A) { int A; }
            ^

llvm-svn: 39257