Correct 'target' default behavior on redecl, allow forward declaration.

Declarations without the attribute were disallowed because it would be
ambiguous which 'target' it was supposed to be on.  For example:

void ___attribute__((target("v1"))) foo();
void foo(); // Redecl of above, or fwd decl of below?
void ___attribute__((target("v2"))) foo();

However, a first declaration doesn't have that problem, and erroring
prevents it from working in cases where the forward declaration is
useful.

Additionally, a forward declaration of target==default wouldn't properly
cause multiversioning, so this patch fixes that.

The patch was not split since the 'default' fix would require
implementing the same check for that case, followed by undoing the same
change for the fwd-decl implementation.

Change-Id: I66f2c5bc2477bcd3f7544b9c16c83ece257077b0
llvm-svn: 347805