These were added in https://github.com/llvm/llvm-project/pull/165803.

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

This allows to set an optional integer level for a given debug type. The
string format is `type[:level]`, and the integer is interpreted as such:

- if not provided: all debugging for this debug type is enabled.
- if >0: all debug that is < to the level is enabled.
- if 0: same as for >0 but also does not disable the other debug-types,
it acts as a negative filter.

The LDBG() macro is updated to accept an optional log level to
illustrate the feature. Here is the expected behavior:

LDBG() << "A"; // Identical to LDBG(1) << "A";
LDBG(2) << "B";

With `--debug-only=some_type`: we'll see A and B in the output.  
With `--debug-only=some_type:1`: we'll see A but not B in the output. 
With `--debug-only=some_type:2`: we'll see A and B in the output. (same
with any level above 2)
With `--debug-only=some_type:0`: we'll see neither A nor B in the
output, but we'll see any other logging for other debug types.

DEBUGLOG_WITH_STREAM_AND_TYPE is an internal implementation detail of
LDBG in DebugLog.h. When NDEBUG is defined,
DEBUGLOG_WITH_STREAM_AND_TYPE is not defined at all.

Signed-off-by: Benoit Jacob <jacob.benoit.1@gmail.com>

Change local variants to use new central one.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.