Extend the IL for selecting TLS models (PR9788)

This allows the user/front-end to specify a model that is better
than what LLVM would choose by default. For example, a variable
might be declared as

  @x = thread_local(initialexec) global i32 42

if it will not be used in a shared library that is dlopen'ed.

If the specified model isn't supported by the target, or if LLVM can
make a better choice, a different model may be used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159077 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AsmParser/LLLexer.cpp

@@ -474,6 +474,9 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(extern_weak);
   KEYWORD(external);
   KEYWORD(thread_local);
+  KEYWORD(localdynamic);
+  KEYWORD(initialexec);
+  KEYWORD(localexec);
   KEYWORD(zeroinitializer);
   KEYWORD(undef);
   KEYWORD(null);