[libcxx] Support building hermetic static library

This is useful when static libc++ library is being linked into
shared libraries that may be used in combination with libraries.
We want to avoid we exporting libc++ symbols in those cases where
this option is useful. This is provided as a CMake option and can
be enabled by libc++ vendors as needed.

Differential Revision: https://reviews.llvm.org/D55404

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@350489 91177308-0d34-0410-b5e6-96231b3b80d8

docs/BuildingLibcxx.rst

@@ -222,6 +222,15 @@ libc++ specific options
 
   Define libc++ destination prefix.
 
+.. option:: LIBCXX_HERMETIC_STATIC_LIBRARY:BOOL
+
+  **Default**: ``OFF``
+
+  Do not export any symbols from the static libc++ library. This is useful when
+  This is useful when the static libc++ library is being linked into shared
+  libraries that may be used in with other shared libraries that use different
+  C++ library. We want to avoid avoid exporting any libc++ symbols in that case.
+
 .. _libc++experimental options:
 
 libc++experimental Specific Options