Extended Description

On Windows, invoking "clang test.c" makes clang hang during startup if %INCLUDE% ends in a \ and presumably some similar conditions as well.

The problem has been described by user qble on http://stackoverflow.com/questions/15992645/clang-compiler-hangs-on-windows

When clang gets invoked with actual work, it assembles a list of arguments and kicks of a new process. Among those arguments is the include environment variable on windows. In case this variable contains spaces or some other (e.g. "C:\Program Files\SomeVendor\Include") or some other special characters, clang adds quotation marks around this string so that it stays a single argument to the new process. If the argument ends in an uneven number (typically one) of backslashes though, the quotation mark becomes escaped and thus instead of a invocation like

clang "C:\Program Files\SomeVendor\Include\" someOtherArg

which makes

argv[1] = "C:\Program Files\SomeVendor\Include\"
argv[2] = "someOtherArg"

it becomes

clang "C:\Program Files\SomeVendor\Include" someOtherArg

which is a unterminated

argv[1] = "C:\Program Files\SomeVendor\Include" someOtherArg ...

The problem can easily be fixed by escaping uneven trailing backslashes by just adding another one. I have added a pach to llvm\lib\Support\Windows which fixes the problem, I didn't feel like messing around with c-strings like the rest of that code does. If you feel strongly about it, I could also write a patch in keeping with the c-style code and without unnecessary copies (though I don't think that code is remotely performance critical)