[mlir][cmake] Do not export MLIR_MAIN_SRC_DIR and MLIR_INCLUDE_DIR (#125842)

MLIR_MAIN_SRC_DIR and MLIR_INCLUDE_DIR point to the source directory,
which is not installed. As such, the installed MLIRConfig.cmake also
should not reference it.

The comment indicates that these are needed for mlir_tablegen(), but I
don't see any related uses.

The motivation for this is the use in flang, where we end up inheriting
a meaningless MLIR_MAIN_SRC_DIR from a previous MLIR build, whose source
directory doesn't exist anymore, and that cannot be overridden with the
correct path, because it's not a cached variable.

Instead do what all the other projects do for LLVM_MAIN_SRC_DIR and
initialize MLIR_MAIN_SRC_DIR to CMAKE_CURRENT_SOURCE_DIR/../mlir.

For MLIR_INCLUDE_DIR there already is an exported MLIR_INCLUDE_DIRS,
which can be used instead.

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