Enable installation of libc++experimental by default.

When libc++experimental was originally created it was empty and therefore there
was no reason to install it. Now that the library contains
<experimental/memory_resource> and <experimental/filesystem> there is a good
reason to install it.

Specifically this patch enables the installation whenever LIBCXX_INSTALL_LIBRARY
is true and LIBCPP_ENABLE_EXPERIMENTAL_LIBRARY is true.

llvm-svn: 280773
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 309a50aefb7840184487b429441d9cc448ab45fb
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 219109e..5e506b8 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -179,7 +179,7 @@
 
 .. option:: LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY:BOOL
 
-  **Default**: ``OFF``
+  **Default**: ``LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY AND LIBCXX_INSTALL_LIBRARY``
 
   Install libc++experimental.a alongside libc++.