[perl #119045] Make list constant mutable again

This broke CPAN.  Maybe we need to rethink this....
[perl #119043] Exempt shared hash key consts from ro

Commit 19130678b4 stopped copy-on-write scalars from being exempt from
readonliness in ck_svconst (called for every OP_CONST).  That was for
the sake of consistency.

It turns out this breaks CPAN modules, so change it back, but only for
shared hash key scalars, not other COW scalars.

This is an historical artefact left over from when copy-on-write used
the read-only flag without the scalar actually being read-only.
Exempt clang from -Wunused-value when run as cc

We already have an exception for it when called as clang or clang-1.2.3,
but not when called as cc or anything else.  Mac OS X Mountain Lion
therefore ends up spitting out lots of warnings, as cc is a symlink
to clang.
[perl #119055] Make qq with no vars read-only

In commit 2484f8db I stopped constant folding from changing the read-
onliness of expressions like 1+2, which should return mutable values,
seeing that ${\1}+2 returns a mutable value.  (After all, constant
folding is supposed to be solely an optimisation, without changing
behaviour.)

This is accomplished by turning on the PADTMP flag, which tells opera-
tors like \ to copy the scalar.

I did not realise at the time that some qq strings like "hello\n" and
qq "foo" (but not just "foo") are implemented using constant folding.
The lexer outputs something like stringify("foo"), which is compiled
down to a stringify op with a constant ("foo") as its only child.

In this case we really do want qq"foo" to be treated as a single con-
stant.  That it is implemented using folding while "foo" is not is an
implementation detail we should hide.

So turn off the PADTMP flag when folding a stringify op.
perldelta for 2f222bbd/#119051 (\&$glob crashing)