[Docs] Update CMake documentation on using ccache

[aidengrossman]

Summary

Since LLVM's minimum CMake version has been bumped to an appropriate
version and CMake noe nwatively supports the
CMAKE_{C,CXX}_COMPILER_LAUNCHER flags, the LLVM_CCACHE_BUILD flag is now
deprecated. This patch adjusts the documentation to reflect this change.

https://discourse.llvm.org/t/llvm-ccache-build-is-deprecated/68431

[aidengrossman]

Ping

[thieta]

Thanks for the patch! In my other patch https://reviews.llvm.org/D143468 (which was reverted) - there was no clear consensus on removing this option. I think that discussion needs to be solved first before we push people to not use LLVM_CCACHE_BUILD.

[aidengrossman]

Ah. Makes sense. I see Alex Bradbury already made a similar suggestion your patch. I should've followed that more closely. Sorry for the noise.

llvm/docs/CMake.rst

 **CMAKE_{C,CXX}_FLAGS**:STRING
   Extra flags to use when compiling C and C++ source files respectively.
 
 **CMAKE_{C,CXX}_COMPILER**:STRING
   Specify the C and C++ compilers to use. If you have multiple
   compilers installed, CMake might not default to the one you wish to
   use.
 
+**CMAKE_{C,CXX}_COMPILER_LAUNCHER**:STRING
+  Specifies the compiler launcher to use. This can be used to do a build with
+  `ccache` by setting `CMAKE_C_COMPILER_LAUNCHER` and
+  `CMAKE_CXX_COMPILER_LAUNCHER` both to `ccache` in addition to having ccache
+  installed.
+
 .. _Frequently Used LLVM-related variables:
 
 Frequently Used LLVM-related variables
 --------------------------------------
 
 The default configuration may not match your requirements. Here are
 LLVM variables that are frequently used to control that. The full
 description is in `LLVM-related variables`_ below.
**LLVM_BUILD_TESTS**:BOOL
   possible to build all unit tests with the target *UnitTests*.
 
 **LLVM_BUILD_TOOLS**:BOOL
   Build LLVM tools. Defaults to ON. Targets for building each tool are generated
   in any case. You can build a tool separately by invoking its target. For
   example, you can build *llvm-as* with a Makefile-based system by executing *make
   llvm-as* at the root of your build directory.
 
-**LLVM_CCACHE_BUILD**:BOOL
-  If enabled and the ``ccache`` program is available, then LLVM will be
-  built using ``ccache`` to speed up rebuilds of LLVM and its components.
-  Defaults to OFF.  The size and location of the cache maintained
-  by ``ccache`` can be adjusted via the LLVM_CCACHE_MAXSIZE and LLVM_CCACHE_DIR
-  options, which are passed to the CCACHE_MAXSIZE and CCACHE_DIR environment
-  variables, respectively.
-
 **LLVM_CREATE_XCODE_TOOLCHAIN**:BOOL
   macOS Only: If enabled CMake will generate a target named
   'install-xcode-toolchain'. This target will create a directory at
   $CMAKE_INSTALL_PREFIX/Toolchains containing an xctoolchain directory which can
   be used to override the default system tools.
 
 **LLVM_DEFAULT_TARGET_TRIPLE**:STRING
   LLVM target to use for code generation when no target is explicitly specified.