Fix for PR2969: generate a memcpy from a constant for constant

Fix for PR2969: generate a memcpy from a constant for constant
initializers.  llvm-gcc appears to be more aggressive, but incorrect,
for constructs like "const int a[] = {1,2,3};"; that said, current
optimizers will do the appropriate optimizations when safe.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60270 91177308-0d34-0410-b5e6-96231b3b80d8