Preprocessor: don't consider // to be a line comment in -E -std=c89 mode.

It's beneficial when compiling to treat // as the start of a line
comment even in -std=c89 mode, since it's not valid C code (with a few
rare exceptions) and is usually intended as such. We emit a pedantic
warning and then continue on as if line comments were enabled.
This has been our behavior for quite some time.

However, people use the preprocessor for things besides C source files.
In today's prompting example, the input contains (unquoted) URLs, which
contain // but should still be preserved.

This change instructs the lexer to treat // as a plain token if Clang is
in C90 mode and generating preprocessed output rather than actually compiling.

<rdar://problem/13338743>