Make Preprocessor::Lex non-recursive.

Before this patch, Lex() would recurse whenever the current lexer changed (e.g.
upon entry into a macro). This patch turns the recursion into a loop: the
various lex routines now don't return a token when the current lexer changes,
and at the top level Preprocessor::Lex() now loops until it finds a token.
Normally, the recursion wouldn't end up being very deep, but the recursion depth
can explode in edge cases like a bunch of consecutive macros which expand to
nothing (like in the testcase test/Preprocessor/macro_expand_empty.c in this
patch).

<rdar://problem/14569770>

llvm-svn: 190980
diff --git a/clang/test/SemaCXX/warn-empty-body.cpp b/clang/test/SemaCXX/warn-empty-body.cpp
index d643ced..d3aaac1 100644
--- a/clang/test/SemaCXX/warn-empty-body.cpp
+++ b/clang/test/SemaCXX/warn-empty-body.cpp
@@ -269,3 +269,8 @@
   test_template<double>(x);
 }
 
+#define IDENTITY(a) a
+void test7(int x, int y) {
+  if (x) IDENTITY(); // no-warning
+}
+