[MSVC] Recognize `static_assert` keyword in C and C++98

Summary:
The main effect is that clang now accepts the following conforming C11
code with MSVC headers:
  #include <assert.h>
  static_assert(1, "true");

This is a non-conforming extension (the keyword is outside the
implementer's namespace), so it is placed under -fms-compatibility
instead of -fms-extensions like most MSVC-specific keyword extensions.

Normally, in C11, the compiler is supposed to provide the _Static_assert
keyword, and assert.h should define static_assert to _Static_assert.
However, that is not what MSVC does, and MSVC doesn't even provide
_Static_assert.

This also has the less important side effect of enabling static_assert
in C++98 mode with -fms-compatibility. It's exceptionally difficult to
use modern MSVC headers without C++14 even, so this is relatively
unimportant.

Fixes PR26672

Patch by Andrey Bokhanko!

Reviewers: rsmith, thakis

Subscribers: cfe-commits, STL_MSFT

Differential Revision: https://reviews.llvm.org/D17444

llvm-svn: 354162
diff --git a/clang/lib/Basic/IdentifierTable.cpp b/clang/lib/Basic/IdentifierTable.cpp
index 3a87f91..7dfcc9c 100644
--- a/clang/lib/Basic/IdentifierTable.cpp
+++ b/clang/lib/Basic/IdentifierTable.cpp
@@ -99,6 +99,7 @@
     KEYMODULES    = 0x100000,
     KEYCXX2A      = 0x200000,
     KEYOPENCLCXX  = 0x400000,
+    KEYMSCOMPAT   = 0x800000,
     KEYALLCXX = KEYCXX | KEYCXX11 | KEYCXX2A,
     KEYALL = (0xffffff & ~KEYNOMS18 &
               ~KEYNOOPENCL) // KEYNOMS18 and KEYNOOPENCL are used to exclude.
@@ -125,6 +126,7 @@
   if (LangOpts.C99 && (Flags & KEYC99)) return KS_Enabled;
   if (LangOpts.GNUKeywords && (Flags & KEYGNU)) return KS_Extension;
   if (LangOpts.MicrosoftExt && (Flags & KEYMS)) return KS_Extension;
+  if (LangOpts.MSVCCompat && (Flags & KEYMSCOMPAT)) return KS_Enabled;
   if (LangOpts.Borland && (Flags & KEYBORLAND)) return KS_Extension;
   if (LangOpts.Bool && (Flags & BOOLSUPPORT)) return KS_Enabled;
   if (LangOpts.Half && (Flags & HALFSUPPORT)) return KS_Enabled;