Clarify how to forward-declare __llvm_profile symbols.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293065 91177308-0d34-0410-b5e6-96231b3b80d8

docs/SourceBasedCodeCoverage.rst

@@ -256,6 +256,8 @@ without using static initializers, do this manually:
   otherwise. Calling this function multiple times appends profile data to an
   existing on-disk raw profile.
 
+In C++ files, declare these as ``extern "C"``.
+
 Collecting coverage reports for the llvm project
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