[PDB] Handle `char` as a builtin type

Summary:
`char`, `signed char` and `unsigned char` are three different types,
and they are mangled differently:

```
void __declspec(dllexport) /* ?foo@@YAXD@Z */ foo(char c) { }
void __declspec(dllexport) /* ?foo@@YAXE@Z */ foo(unsigned char c) { }
void __declspec(dllexport) /* ?foo@@YAXC@Z */ foo(signed char c) { }
```

This commit separates `char` from `signed char` and `unsigned char`.

Reviewers: asmith, zturner, labath

Reviewed By: asmith, zturner

Subscribers: teemperor, lldb-commits, stella.stamenova

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D52468

llvm-svn: 343298