Do not enforce absolute path argv0 in windows

Even if we support no-canonical-prefix on
clang-cl(https://reviews.llvm.org/D47480), argv0 becomes absolute path
in clang-cl and that embeds absolute path in /showIncludes.

This patch removes such full path normalization from InitLLVM on
windows, and that removes absolute path from clang-cl output
(obj/stdout/stderr) when debug flag is disabled.

Patch by Takuto Ikuta!

Differential Revision https://reviews.llvm.org/D47578

llvm-svn: 334602