[libc++] Integrate the PSTL into libc++

Summary:
This commit allows specifying LIBCXX_ENABLE_PARALLEL_ALGORITHMS when
configuring libc++ in CMake. When that option is enabled, libc++ will
assume that the PSTL can be found somewhere on the CMake module path,
and it will provide the C++17 parallel algorithms based on the PSTL
(that is assumed to be available).

The commit also adds support for running the PSTL tests as part of
the libc++ test suite.

Reviewers: rodgert, EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits, mclow.lists, EricWF

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60480

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@366593 91177308-0d34-0410-b5e6-96231b3b80d8

include/module.modulemap

@@ -275,6 +275,10 @@ module std [system] {
     header "exception"
     export *
   }
+  module execution {
+    header "execution"
+    export *
+  }
   module filesystem {
     header "filesystem"
     export *