[PR33394] Avoid lexing editor placeholders when Clang is used only

for preprocessing

r300667 added support for editor placeholder to Clang. That commit didn’t take
into account that users who use Clang for preprocessing only (-E) will get the
"editor placeholder in source file" error when preprocessing their source
(PR33394). This commit ensures that Clang doesn't lex editor placeholders when
running a preprocessor only action.

rdar://32718000

Differential Revision: https://reviews.llvm.org/D34256

llvm-svn: 305576