[lit] Update internal shell lexer to handle LLDB persistent vars. (llvm#156125)

LLDB allows creation of 'persistent' variables, with names that start
with '$'. The lit internal shell was escaping the '$', making it '\\$',
in some CHECK lines, which causes an LLDB test,
TestExprWithSideEffectOnConvenienceVar, to fail when using the lit
internal shell.

Further explanation of the failing LLDB test: LLDB convenience variables
start with '$'. The test passes several quoted commands that use and
update convenience variables to lldb as arguments to be run in batch
mode. The tool that packages up the complete string and passes it to the
lit internal shell lexer for lexing inserts a backslash in front of the
'$' before passing the string in for lexing. The lexer was passing this
change along, causing the tests to fail.

This PR fixes the issue by having the lexer remove the newly added
escape on the '$'.

Author: cmtice

llvm/utils/lit/lit/ShUtil.py

@@ -115,6 +115,11 @@ def lex_arg_quoted(self, delim):
             c = self.eat()
             if c == delim:
                 return str
+            # LLDB uses "$" at the start of global variable names; it should
+            # not be escaped nor dropped.
+            elif c == "\\" and self.look() == "$":
+                c = self.eat()
+                str += c
             elif c == "\\" and delim == '"':
                 # Inside a '"' quoted string, '\\' only escapes the quote
                 # character and backslash, otherwise it is preserved.

llvm/utils/lit/tests/unit/ShUtil.py

@@ -28,7 +28,8 @@ def test_quoting(self):
         self.assertEqual(self.lex(""" a\\ b a\\\\b """), ["a b", "a\\b"])
         self.assertEqual(self.lex(""" "" "" """), ["", ""])
         self.assertEqual(self.lex(""" a\\ b """, win32Escapes=True), ["a\\", "b"])
-
+        self.assertEqual(self.lex('"\\$y = 11"'), ["$y = 11"])
+        self.assertEqual(self.lex('"expr \\$y = 11"'), ["expr $y = 11"])
 
 class TestShParse(unittest.TestCase):
     def parse(self, str):