Lit: Allow overriding llvm tool paths+arguments, make -D an alias for --param

[MatzeB]

Summary

These changes allow usages where you want to pass an additional
commandline option to all invocations of a specific llvm tool. Example:

llvm-lit -Dllc=llc -enable-misched -verify-machineinstrs

[• ddunbar]

Fine by me, can you also update the 'lit' docs in CommandGuide to note this.

llvm/trunk/docs/CommandGuide/lit.rst

  Run ``N`` tests in parallel.  By default, this is automatically chosen to
  match the number of detected available CPUs.
 
 .. option:: --config-prefix=NAME
 
  Search for :file:`{NAME}.cfg` and :file:`{NAME}.site.cfg` when searching for
  test suites, instead of :file:`lit.cfg` and :file:`lit.site.cfg`.
 
-.. option:: --param NAME, --param NAME=VALUE
+.. option:: -D NAME, -D NAME=VALUE, --param NAME, --param NAME=VALUE
 
  Add a user defined parameter ``NAME`` with the given ``VALUE`` (or the empty
  string if not given).  The meaning and use of these parameters is test suite
  dependent.
 
 .. _output-options:
 
 OUTPUT OPTIONS

llvm/trunk/test/lit.cfg

for pattern in [r"\bbugpoint\b(?!-)",
     # Extract the tool name from the pattern.  This relies on the tool
     # name being surrounded by \b word match operators.  If the
     # pattern starts with "| ", include it in the string to be
     # substituted.
     tool_match = re.match(r"^(\\)?((\| )?)\W+b([0-9A-Za-z-_]+)\\b\W*$",
                           pattern)
     tool_pipe = tool_match.group(2)
     tool_name = tool_match.group(4)
+    # Did the user specify the tool path + arguments? This allows things like
+    # llvm-lit "-Dllc=llc -enable-misched -verify-machineinstrs"
+    tool_path = lit_config.params.get(tool_name)
+    if tool_path is None:
         tool_path = lit.util.which(tool_name, llvm_tools_dir)
-    if not tool_path:
+    if tool_path is None:
         # Warn, but still provide a substitution.
         lit_config.note('Did not find ' + tool_name + ' in ' + llvm_tools_dir)
         tool_path = llvm_tools_dir + '/' + tool_name
     if (tool_name == "llc" and
        'LLVM_ENABLE_MACHINE_VERIFIER' in os.environ and
        os.environ['LLVM_ENABLE_MACHINE_VERIFIER'] == "1"):
         tool_path += " -verify-machineinstrs"
     config.substitutions.append((pattern, tool_pipe + tool_path))

llvm/trunk/utils/lit/lit/main.py

parser.add_option("", "--version", dest="show_version",
                       help="Show version and exit",
                       action="store_true", default=False)
     parser.add_option("-j", "--threads", dest="numThreads", metavar="N",
                       help="Number of testing threads",
                       type=int, action="store", default=None)
     parser.add_option("", "--config-prefix", dest="configPrefix",
                       metavar="NAME", help="Prefix for 'lit' config files",
                       action="store", default=None)
-    parser.add_option("", "--param", dest="userParameters",
+    parser.add_option("-D", "--param", dest="userParameters",
                       metavar="NAME=VAL",
                       help="Add 'NAME' = 'VAL' to the user defined parameters",
                       type=str, action="append", default=[])
 
     group = OptionGroup(parser, "Output Format")
     # FIXME: I find these names very confusing, although I like the
     # functionality.
     group.add_option("-q", "--quiet", dest="quiet",