On Windows, there are multiple variants of the C runtime that must be
explicitly specified and consistently used from the runtime to the
application. The new `-libc` option allows us to control the linking
phase by correctly embedding the requested library to be linked. It is
made into a required parameter on Windows and will add in the
appropriate flags for the imported C headers as well. This ensures that
the C library is not incorrectly linked.
The object file extension on Windows is `.obj` rather than `.o`. Ensure
that we get the extension correct when compiling for Windows.
Furthermore, ensure that we install with the correct extension when
installing a cross-compiled image. As we may be using the homegrown
cross-compilation system, we must explicitly handle the extension
ourselves. This allows nearly a 100 additional tests to pass on
Windows.
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.
