[codeview] Use character types for all byte-sized integer types

The VS debugger doesn't appear to understand the 0x68 or 0x69 type
indices, which were probably intended for use on a platform where a C
'int' is 8 bits. So, use the character types instead. Clang was already
using the character types because '[u]int8_t' is usually defined in
terms of 'char'.

See the Rust issue for screenshots of what VS does:
https://github.com/rust-lang/rust/issues/36646

Fixes PR30552

llvm-svn: 282739