Build llvm with ccache if package is present

This patch has the following changes

The CMake variable LLVM_CCACHE_BUILD is set to OFF by default.
Set this to ON for a ccache enabled build

CCACHE_CPP2 is required to compile the source file directly instead
of compiling the preprocessed file. This will help WERROR is turned ON
for a host clang compiler

The below two options makes more sense in the context of a buildbot

CCACHE_HASHDIR is required to maintain the separate cached data across
builders. This will also help the debuggers to point to the correct source
location

CCACHE_SIZE is important in the perspective of buildbot to increase the
limit on the amount of data to hold in cache for faster compilation

CCACHE_DIR is used to save the cached data to a specific directory.

llvm-svn: 277389