[Clang] Allow raw string literals in C as an extension (#88265)

This enables raw R"" string literals in C in some language modes
and adds an option to disable or enable them explicitly as an
exten

[Clang] Allow raw string literals in C as an extension (#88265)

This enables raw R"" string literals in C in some language modes
and adds an option to disable or enable them explicitly as an
extension.

Background: GCC supports raw string literals in C in `-gnuXY` modes
starting with gnu99. This pr both enables raw string literals in gnu99
mode and later in C and adds an `-f[no-]raw-string-literals` flag to override
this behaviour. The decision not to enable raw string literals in gnu89
mode, according to the GCC devs, is intentional as that mode is supposed
to be used for ‘old code’ that they don’t want to break; we’ve decided to
match GCC’s behaviour here as well.

The `-fraw-string-literals` flag can additionally be used to enable raw string
literals in modes where they aren’t enabled by default (such as c99—as
opposed to gnu99—or even e.g. C++03); conversely, the negated flag can
be used to disable them in any gnuXY modes that *do* provide them by
default, or to override a previous flag. However, we do *not* support
disabling raw string literals (or indeed either of these two options) in
C++11 mode and later, because we don’t want to just start supporting
disabling features that are actually part of the language in the general case.

This fixes #85703.

show more ...