lit: Let lit.util.which() return a normcase()ed path

LLVMConfig.with_environment() uses os.path.normcase(os.path.normpath(x)) to
normalize temporary env vars. LLVMConfig.use_clang() uses with_environment() to
temporarily set PATH and then look for clang there. This means that on Windows,
clang will be run with a path like c:\foo\bin\clang.EXE (with a lower-case
"C:").

lit.util.which() used to not do this, which means the executables added in
clang/test/lit.cfg.py (e.g. c-index-test) were run with a path like
C:\foo\bin\c-index-test.EXE (because both CMake and GN happen to write
clang_tools_dir with an upper-case C to lit.site.cfg.py).

clang/test/Index/pch-from-libclang.c requires that both c-index-test and clang
use _exactly_ the same resource dir path (same case and everything), because a
hash of the resource directory is used as module cache path.

This patch is necessary but not sufficient to make pch-from-libclang.c pass on
Windows.

Differential Revision: https://reviews.llvm.org/D57343

llvm-svn: 352704