[llvm] [Coverage] getMaxBitmapSize: Scan `max(BitmapIdx)` instead of the last `Decision` (PR #78963)

[Alan Phipps via llvm-commits]

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+# Test the assertion failure in llvm-cov with empty bitmap (#78922)
+# REQUIRES: asserts
+
+# RUN: llvm-profdata merge %S/Inputs/mcdc-maxbs.proftext -o %t.profdata
+# RUN: llvm-cov report --show-mcdc-summary %S/Inputs/mcdc-maxbs.o -instr-profile %t.profdata
+
+# Instructions for regenerating the test object:
+
+clang -O3 -fcoverage-mcdc -fprofile-instr-generate \
+    -fcoverage-mapping -fcoverage-compilation-dir=. \
+    mcdc-maxbs.c -c -o mcdc-maxbs.o
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evodius96 wrote:

You may want to note `cd $S/Inputs `to underscore the location of mcdc-maxbs.c

https://github.com/llvm/llvm-project/pull/78963