Prohibit use of _ExtInt in atomic intrinsic

The _ExtInt type allows custom width integers, but the atomic memory
access's operand must have a power-of-two size. _ExtInts with
non-power-of-two size should not be allowed for atomic intrinsic.

Before this change:

$ cat test.c

typedef unsigned _ExtInt(42) dtype;
void verify_binary_op_nand(dtype* pval1, dtype val2)
{    __sync_nand_and_fetch(pval1, val2); }

$ clang test.c

clang-11:
/home/ubuntu/llvm_workspace/llvm/clang/lib/CodeGen/CGBuiltin.cpp:117:
llvm::Value*
EmitToInt(clang::CodeGen::CodeGenFunction&, llvm::Value*,
clang::QualType, llvm::IntegerType*): Assertion `V->getType() ==
IntType' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the
crash backtrace, preprocessed source, and associated run script.

After this change:

$ clang test.c

test.c:3:30: error: Atomic memory operand must have a power-of-two size
{    __sync_nand_and_fetch(pval1, val2); }
^

List of the atomic intrinsics that have this
problem:

__sync_fetch_and_add
__sync_fetch_and_sub
__sync_fetch_and_or
__sync_fetch_and_and
__sync_fetch_and_xor
__sync_fetch_and_nand
__sync_nand_and_fetch
__sync_and_and_fetch
__sync_add_and_fetch
__sync_sub_and_fetch
__sync_or_and_fetch
__sync_xor_and_fetch
__sync_fetch_and_min
__sync_fetch_and_max
__sync_fetch_and_umin
__sync_fetch_and_umax
__sync_val_compare_and_swap
__sync_bool_compare_and_swap

Differential Revision: https://reviews.llvm.org/D83340