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>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190980 91177308-0d34-0410-b5e6-96231b3b80d8