[CMake] Add LLVM runtimes directory

Summary:
There are a few LLVM projects that produce runtime libraries. Ideally
runtime libraries should be built differently than other projects,
specifically they should be built using the just-built toolchain.

There is support for building compiler-rt in this way from the clang
build. Moving this logic into the LLVM build is interesting because it
provides a simpler way to extend the just-built toolchain to include
LLD and the LLVM object file tools.

Once this functionality is better fleshed out and tested we’ll want to
encapsulate it in a module that can be used for clang standalone
builds, and we’ll want to make it the default way to build compiler-rt.

With this patch applied there is no immediate change in the build.
Moving compiler-rt out from llvm/projects into llvm/runtimes enables
the functionality.

This code has a few improvements over the method provided by
LLVM_BUILD_EXTERNAL_COMPILER_RT. Specifically the sub-ninja command is
always invoked, so changes to compiler-rt source files will get built
properly, so this patch can be used for iterative development with
just-built tools.

This first patch only works with compiler-rt. Support for other
runtime projects will be coming in follow-up patches.

Reviewers: chandlerc, bogner

Subscribers: kubabrecka, llvm-commits

Differential Revision: http://reviews.llvm.org/D20992

llvm-svn: 273620

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/.gitignore b/llvm/.gitignore
index e3d191d..8144deb 100644
--- a/llvm/.gitignore
+++ b/llvm/.gitignore
@@ -46,6 +46,8 @@ autoconf/autom4te.cache
 projects/*
 !projects/*.*
 !projects/Makefile
+runtimes/*
+!runtimes/*.*
 # Clang, which is tracked independently.
 tools/clang
 # LLDB, which is tracked independently.