This fixes an issue that, when you start lldb or use `target create`
with a program name which is on $PATH, or not specify the .exe suffix of
a program in the working directory on Windows, you get a confusing
error, for example:
(lldb) target create notepad
error: 'C:\WINDOWS\SYSTEM32\notepad.exe' doesn't contain any 'host'
platform architectures: i686, x86_64, i386, i386
Fixes https://github.com/mstorsjo/llvm-mingw/issues/265
Reviewed By: DavidSpickett
Differential Revision: https://reviews.llvm.org/D127436