[lldb] Resolve exe location for `target create`

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