Use off-the-shelf ccache enablement

[vhscampos]

LLVM's CMake system offers an off-the-shelf detection and enablement of ccache in its builds: LLVM_CCACHE_BUILD.

This patch replaces our manually written mechanism by the one that LLVM offers.

[ostannard]

LGTM

[statham-arm]

What's the effect of this on subsidiary builds like libc and libc++? I guess neither method would pass through a request to use ccache, so this doesn't make anything worse?

[statham-arm]

... yes, discussing this elsewhere, we think that ccache was not used for library builds, before or after this change. So I'm happy that this is an unconditional improvement.

(@vhscampos also points out that ccache may do the wrong thing if the compiler has changed, so that if you're rebuilding the whole toolchain, you don't want to ccache the library builds. So that might have been a deliberate choice, although it doesn't suit everybody -- library developers who aren't constantly changing the compiler would probably prefer more ccache.)

[dcandler]

Our CMake did originally use the LLVM_CCACHE_BUILD option, but it was going to be deprecated: https://discourse.llvm.org/t/llvm-ccache-build-is-deprecated/68431, which led to us removing it: #196

The upstream removal of the option was reverted to give more time for notice, and I'm not sure if it will still happen, but the motivation behind it remains. In particular LLVM_CCACHE_BUILD is only an on/off option and doesn't allow for different programs, which is why it was felt it was better to use the options built into CMake rather than a custom solution in LLVM.

[vhscampos]

Thanks. Since LLVM_CCACHE_BUILD is in the path of deprecation, there's no point in this PR.