Revert r44626, which turned off the use of readonly

Revert r44626, which turned off the use of readonly
and readnone for functions with bodies because it
broke llvm-gcc-4.2 bootstrap.  It turns out that,
because of LLVM's array_ref hack, gcc was computing
pure/const attributes wrong (now fixed by turning
off the gcc ipa-pure-const pass).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@44937 91177308-0d34-0410-b5e6-96231b3b80d8