[clang-cl] Fix cross-compilation with MSVC 2017.

[clang-cl] Fix cross-compilation with MSVC 2017.

clang-cl works best when the user runs vcvarsall to set up
an environment before running, but even this is not enough
on VC 2017 when cross compiling (e.g. using an x64 toolchain
to target x86, or vice versa).

The reason is that although clang-cl itself will have a
valid environment, it will shell out to other tools (such
as link.exe) which may not.  Generally we solve this through
adding the appropriate linker flags, but this is not enough
in VC 2017.

The cross-linker and the regular linker both link against
some common DLLs, but these DLLs live in the binary directory
of the native linker.  When setting up a cross-compilation
environment through vcvarsall, it will add *both* directories
to %PATH%, so that when cl shells out to any of the associated
tools, those tools will be able to find all of the dependencies
that it links against.  If you don't do this, link.exe will
fail to run because the loader won't be able to find all of
the required DLLs that it links against.

To solve this we teach the driver how to spawn a process with
an explicitly specified environment.  Then we modify the
PATH before shelling out to subtools and run with the modified
PATH.

Patch by Hamza Sood
Differential Revision: https://reviews.llvm.org/D30991

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298098 91177308-0d34-0410-b5e6-96231b3b80d8