C++, fix parsing of full xrefs.

[jakobandersen]

If a full xref has a short xref as prefix, e.g., T f(), parsing would fail.

Relates

• Fixes Cross-referencing a function that returns a pointer with :cpp:func: causes error "Invalid definition: Expected end of definition" #6208

[tk0miya]

Is this critical bug?

After 2.0 release, we've changed branch model:

sphinx/CONTRIBUTING.rst

Lines 323 to 345 in 44c49f4

	``master``
	Development for MAJOR version.
	All changes including incompatible behaviors and public API updates are
	allowed.
	``X.Y``
	Where ``X.Y`` is the ``MAJOR.MINOR`` release. Used to maintain current
	MINOR release. All changes are allowed if the change preserves
	backwards-compatibility of API and features.
	Only the most recent ``MAJOR.MINOR`` branch is currently retained. When a
	new MAJOR version is released, the old ``MAJOR.MINOR`` branch will be
	deleted and replaced by an equivalent tag.
	``X.Y.Z``
	Where ``X.Y.Z`` is the ``MAJOR.MINOR.PATCH`` release. Only
	backwards-compatible bug fixes are allowed. In Sphinx project, PATCH
	version is used for urgent bug fix.
	``MAJOR.MINOR.PATCH`` branch will be branched from the ``v`` prefixed
	release tag (ex. make 2.3.1 that branched from v2.3.0) when a urgent
	release is needed. When new PATCH version is released, the branch will be
	deleted and replaced by an equivalent tag (ex. v2.3.1).

Therefore, 2.0.1 is much better for critical bug fixes.

[codecov]

Codecov Report

Merging #6226 into 2.0.1 will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            2.0.1    #6226      +/-   ##
==========================================
+ Coverage   83.54%   83.58%   +0.03%     
==========================================
  Files         272      272              
  Lines       39357    39369      +12     
  Branches     5868     5868              
==========================================
+ Hits        32882    32907      +25     
+ Misses       5126     5113      -13     
  Partials     1349     1349

Impacted Files	Coverage Δ
sphinx/domains/cpp.py	82.91% <100%> (+0.28%)	⬆️
tests/test_domain_cpp.py	96.21% <100%> (+0.07%)	⬆️

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[jakobandersen]

@tk0miya, thanks. It's not everything-is-broken urgent, but I have rebased it to the 2.0.1 branch to get it released sooner.