[Coverage] Introduce "partial fold" on BranchRegion (#112694)

Currently both True/False counts were folded. It lost the information,
"It is True or False before folding." It prevented recalling branch
counts in merging template instantiations.

In `llvm-cov`, a folded branch is shown as:

- `[True: n, Folded]`
- `[Folded, False n]`

In the case If `n` is zero, a branch is reported as "uncovered". This is
distinguished from "folded" branch. When folded branches are merged,
`Folded` may be dissolved.

In the coverage map, either `Counter` is `Zero`. Currently both were
`Zero`.

Since "partial fold" has been introduced, either case in `switch` is
omitted as `Folded`.

Each `case:` in `switch` is reported as `[True: n, Folded]`, since
`False` count doesn't show meaningful value.

When `switch` doesn't have `default:`, `switch (Cond)` is reported as
`[Folded, False: n]`, since `True` count was just the sum of `case`(s).
`switch` with `default` can be considered as "the statement that doesn't
have any `False`(s)".

1 files changed, 2 insertions, 2 deletions

diff --git a/clang/test/CoverageMapping/mcdc-system-headers.cpp b/clang/test/CoverageMapping/mcdc-system-headers.cpp
index ae26ed5fe469..cb1c8743c36e 100644
--- a/clang/test/CoverageMapping/mcdc-system-headers.cpp
+++ b/clang/test/CoverageMapping/mcdc-system-headers.cpp
@@ -17,10 +17,10 @@
 int func0(int a) {
   // CHECK: Decision,File 0, [[@LINE+3]]:11 -> [[@LINE+3]]:21 = M:3, C:2
   // W_SYS: Expansion,File 0, [[@LINE+2]]:11 -> [[@LINE+2]]:16 = #0 (Expanded file = 1)
-  // X_SYS: Branch,File 0, [[@LINE+1]]:11 -> [[@LINE+1]]:11 = 0, 0 [1,2,0]
+  // X_SYS: Branch,File 0, [[@LINE+1]]:11 -> [[@LINE+1]]:11 = #1, 0 [1,2,0]
   return (CONST && a);
   // CHECK: Branch,File 0, [[@LINE-1]]:20 -> [[@LINE-1]]:21 = #2, (#1 - #2) [2,0,0]
-  // W_SYS: Branch,File 1, [[@LINE-16]]:15 -> [[@LINE-16]]:17 = 0, 0 [1,2,0]
+  // W_SYS: Branch,File 1, [[@LINE-16]]:15 -> [[@LINE-16]]:17 = #1, 0 [1,2,0]
 }
 
 // CHECK: _Z5func1ii: