[Modules] Add 'no_undeclared_includes' module map attribute

The 'no_undeclared_includes' attribute should be used in a module to
tell that only non-modular headers and headers from used modules are
accepted.

The main motivation behind this is to prevent dep cycles between system
libraries (such as darwin) and libc++.

Patch by Richard Smith!

llvm-svn: 284797

1 files changed, 1 insertions, 0 deletions

diff --git a/clang/test/Modules/Inputs/libc-libcxx/include/math.h b/clang/test/Modules/Inputs/libc-libcxx/include/math.h
new file mode 100644
index 00000000000..380ab41ca0a
--- /dev/null
+++ b/clang/test/Modules/Inputs/libc-libcxx/include/math.h
@@ -0,0 +1 @@
+int abs(int);