[CMake][runtimes] Support monorepo layout with runtimes build

We introduce a new variable LLVM_ENABLE_RUNTIMES which works
similarly to LLVM_ENABLE_PROJECTS and allows specifying runtimes
that will be enabled in the runtimes build.

Differential Revision: https://reviews.llvm.org/D40233

llvm-svn: 319107
GitOrigin-RevId: a08d65ded2237fb69d769410e7a4086b00307a5a
1 file changed