Fix or suppress GCC warnings during build.

Summary:
Currently a number of GCC warnings are emitted when building libc++. This patch fixes or ignores all of them. The primary changes are:

* Work around strict aliasing issues in `typeinfo::hash_code()` by using __attribute__((may_alias)). However I think a non-aliasing `hash_code()` implementation is possible. Further investigation needed.
* Add `_LIBCPP_UNREACHABLE()` to switch in `strstream.cpp` to avoid -Wpotentially-uninitialized.
* Fix -Wunused-value warning in `__all` by adding a void cast.
* Ignore -Wattributes for now. There are a number of real attribute issues when using GCC but enabling the warning is too noisy.
* Ignore -Wliteral-suffix since it warns about the use of reserved identifiers. Note Only GCC 7.0 supports disabling this warning.
* Ignore -Wc++14-compat since it warns about the sized new/delete overloads.



Reviewers: EricWF

Differential Revision: https://reviews.llvm.org/D24003

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@280007 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 64c6c31..d0ed9df 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -322,10 +322,19 @@
 # Warning flags ===============================================================
 add_definitions(-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 add_compile_flags_if_supported(
-    -Wall -W -Wwrite-strings
-    -Wno-unused-parameter -Wno-long-long -Wno-user-defined-literals
-    -Wno-covered-switch-default
+    -Wall -Wextra -W -Wwrite-strings
+    -Wno-unused-parameter -Wno-long-long
     -Werror=return-type)
+if ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
+    add_compile_flags_if_supported(
+        -Wno-user-defined-literals
+        -Wno-covered-switch-default)
+elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
+    add_compile_flags_if_supported(
+        -Wno-attributes # FIXME: Fix -Wattribute warnings.
+        -Wno-literal-suffix
+        -Wno-c++14-compat)
+endif()
 if (LIBCXX_ENABLE_WERROR)
   add_compile_flags_if_supported(-Werror)
   add_compile_flags_if_supported(-WX)