Support sccache and ccache

[kj4ezj]

This pull request modifies LLVM's CMake to automatically detect the presence of sccache and use it to accelerate compiling and linking. If sccache is not found, CMake will look for ccache to use for compiling and linking. If neither sccache nor ccache are found on the host, behavior will be unchanged.

This change is necessary to implement sccache on the eosio-dot-cdt CI/CD pipeline, which will allow us to substantially reduce build times.

Tested

I have tested these changes on my local machine which has the following environment...

llvm$ sw_vers
ProductName:	Mac OS X
ProductVersion:	10.14.5
BuildVersion:	18F132
llvm$ cmake --version
cmake version 3.14.1

CMake suite maintained and supported by Kitware (kitware.com/cmake).
llvm$ ls /usr/local/Cellar/boost/
1.68.0_1
llvm$ cat /usr/local/Cellar/boost/*/include/boost/version.hpp | grep -P '^ *#define +BOOST_VERSION +'
#define BOOST_VERSION 106800
llvm$ ccache --version
-bash: ccache: command not found
llvm$ sccache --version
-bash: sccache: command not found

...using these steps:

llvm$ mkdir build
llvm$ cd build
build$ cmake ..
build$ make -j 10

Everything worked.