Previously, the TSan runtime only required libdispatch on Darwin, which
required no explicit linker flags, because libdispatch is always
provided by the system (libSystem).
Now, the TSan runtime also has a link-time dependency on libdispatch on
Linux, where libdispatch (and the blocks runtime) is just another
library. We therefore need to specify them as additonal linker options.
In the Darwin toolchain the linker is invoked directly, and compiler_rt
is used if it is found, but in Unix platforms, clang++ is invoked
instead, and the clang driver will invoke the linker. Howerver there was
no way of modifying this clang++ invocation, so there's no way of
providing `--rtlib=` and change the platform default (which is normally
libgcc). The only workaround is doing the work that the Swift driver is
doing "manually".
The change adds a new option (with help hidden, but we can change that)
to allow providing extra arguments to the clang++ invocation. The change
is done in the two places in the Unix and Windows toolchains that I
found the clang driver was being used.
Includes some simple tests.
This change allows the swift driver to link the ubsan runtime if
`-sanitize=undefined` is specified.
This is useful for sanitizing linked Objective-C code.
This enables response files for any jobs that invoke `swift` or another
toolchain tool that goes through the same driver code path, like
`swift-autolink-extract`.
Previously extra linker arguments had different behavior on darwin vs
other unix platforms. On darwin the arguments passed with -Xlinker would
be passed to the linker before the default arguments, where as with the
default unix toolchain they would be passed afterwards.
There isn't really a great option for which order these should be in.
If you want to have a custom rpath that takes precedence over the
default rpaths, you want them to be passed before, but if you want to
negate a default argument you want them to come after.
This change unifies the behavior so at least you always get the same
behavior across platforms.
