[lldb/win] Fix TestIRMemoryMapWindows.test when running tests in git bash

[thakis]

Summary

lit.util.which('link') picks up the wrong link.exe in git bash, leading
to this error:

# command stderr:
/usr/bin/link: extra operand '/LIBPATH:C:\\Program Files (x86)\\Windows Kits\\10\\lib\\10.0.17763.0\\ucrt\\x64'
Try '/usr/bin/link --help' for more information.

Instead, assume that link.exe is next to cl.exe.

[amccarth]

LGTM.

lldb/test/Shell/helper/toolchain.py

def use_lldb_substitutions(config):
 
     llvm_config.add_tool_substitutions(primary_tools, [config.lldb_tools_dir])
 
 def _use_msvc_substitutions(config):
     # If running from a Visual Studio Command prompt (e.g. vcvars), this will
     # detect the include and lib paths, and find cl.exe and link.exe and create
     # substitutions for each of them that explicitly specify /I and /L paths
     cl = lit.util.which('cl')
-    link = lit.util.which('link')
 
-    if not cl or not link:
+    if not cl:
         return
 
+    # Don't use lit.util.which() for link.exe: In `git bash`, it will pick
+    # up /usr/bin/link (another name for ln).
+    link = os.path.join(os.path.dirname(cl), 'link.exe')
+
     cl = '"' + cl + '"'
     link = '"' + link + '"'
     includes = os.getenv('INCLUDE', '').split(';')
     libs = os.getenv('LIB', '').split(';')
 
     config.available_features.add('msvc')
     compiler_flags = ['"/I{}"'.format(x) for x in includes if os.path.exists(x)]
     linker_flags = ['"/LIBPATH:{}"'.format(x) for x in libs if os.path.exists(x)]