Produce a better diagnostic for global register variables.

Currently, when there is a global register variable in a program that
is bound to an invalid register, clang/llvm prints an error message that
is not very user-friendly.

This commit improves the diagnostic and moves the check that used to be
in the backend to Sema. In addition, it makes changes to error out if
the size of the register doesn't match the declared variable size.

e.g., volatile register int B asm ("rbp");

rdar://problem/23084219

Differential Revision: http://reviews.llvm.org/D13834

llvm-svn: 253405