[lit] Update internal shell lexer to handle LLDB persistent vars.

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 some LLDB tests
to fail when using the lit internal shell.

This PR fixes that by having the lexer remove the 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.