[lldb] Don't check environment default char signedness when creating clang type for "char"

With -f(un)signed-char, the die corresponding to "char" may be the opposite DW_ATE_(un)signed_char from the default platform signedness.
Ultimately we should determine whether a type is the unspecified signedness char by looking if its name is "char" (as opposed to "signed char"/"unsigned char") and not care about DW_ATE_(un)signed_char matching the platform default.

Fixes #23443

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D136011