[CMake] Add support for specifying arguments to the bootstrap build.

This adds support for three types of argument specifications for bootstrap builds:

(1) Arguments prefixed with BOOTSTRAP_* will be passed through with the leading BOOTSTRAP_ removed.
(2) CLANG_BOOTSTRAP_PASSTHROUGH can specify a list of variables to be passed through as they are set.
(3) BOOTSTRAP_DEFAULT_PASSTHROUGH is a list of some default passthrough variables that are always passed through. Those variables include the version string and should only specify variables that are always expected to be the same between the stage1 and stage2

llvm-svn: 253721