Add _LIBCPP_COMPILER_[CLANG|GCC|MSVC|IBM] macros.

This patch refactors the compiler detection done in `__config` by creating a
set of `_LIBCPP_COMPILER_<TYPE>` macros. The goal of this patch is to make
it easier to detect what compiler is being used outside of `__config`.

Additionally this patch removes workarounds for GCC in `__bit_reference`. I
tested GCC 4.8 and 4.9 without the workaround and neither seemed to need it
anymore.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291286 91177308-0d34-0410-b5e6-96231b3b80d8

src/locale.cpp

@@ -140,7 +140,7 @@ class _LIBCPP_HIDDEN locale::__imp
     : public facet
 {
     enum {N = 28};
-#if defined(_LIBCPP_MSVC)
+#if defined(_LIBCPP_COMPILER_MSVC)
 // FIXME: MSVC doesn't support aligned parameters by value.
 // I can't get the __sso_allocator to work here
 // for MSVC I think for this reason.