This pull request adds support for parsing the source language in both
DWARF and CodeView. Specifically,

- The `LVSourceLanguage` class is introduced to represent any supported
language by any of the debug info representations.

- Update `LVDWARFReader.cpp` and `LVCodeViewVisitor.cpp` to parse the
source language where it applies. Added a new `=Language` attribute;
`getAttributeLanguage()` is internally used to control whether this
information is being printed.

This PR was split from https://github.com/llvm/llvm-project/pull/137228
(which introduced support for `DW_TAG_module` and `DW_AT_byte_size`).

This PR improves `LVDWARFReader` by introducing handling of
`DW_AT_byte_size`. Most DWARF emitters include this attribute for types
to specify the size of an entity of the given type.

Identified with misc-include-cleaner.

I'm planning to remove StringRef::equals in favor of
StringRef::operator==.

- StringRef::operator==/!= outnumber StringRef::equals by a factor of
  53 under llvm/ in terms of their usage.

- The elimination of StringRef::equals brings StringRef closer to
  std::string_view, which has operator== but not equals.

- S == "foo" is more readable than S.equals("foo"), especially for
  !Long.Expression.equals("str") vs Long.Expression != "str".

Note that llvm::support::endianness has been renamed to
llvm::endianness while becoming an enum class as opposed to an enum.
This patch replaces llvm::support::{big,little,native} with
llvm::endianness::{big,little,native}.

The `S_INLINEES` debug symbol is used to record all the functions that
are directly inlined within the current function (nested inlining is
ignored).

This change implements support for emitting the `S_INLINEES` debug
symbol in LLVM, and cleans up how the `S_INLINEES` and `S_CALLEES` debug
symbols are dumped.