llvm-cov: Implement the preserve-paths flag

Until now, when a path in a gcno file included a directory, we would
emit our .gcov file in that directory, whereas gcov always emits the
file in the current directory. In doing so, this implements gcov's
strange name-mangling -p flag, which is needed to avoid clobbering
files when two with the same name exist in different directories.

The path mangling is a bit ugly and only handles unix-like paths, but
it's simple, and it doesn't make any guesses as to how it should
behave outside of what gcov documents. If we decide this should be
cross platform later, we can consider the compatibility implications
then.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200754 91177308-0d34-0410-b5e6-96231b3b80d8

test/tools/llvm-cov/Inputs/test_paths.cpp.gcov

@@ -0,0 +1,79 @@
+        -:    0:Source:srcdir/./nested_dir/../test.cpp
+        -:    0:Graph:test_paths.gcno
+        -:    0:Data:test_paths.gcda
+        -:    0:Runs:3
+        -:    0:Programs:1
+        -:    1:#include "test.h"
+        -:    2:#include <cstdlib>
+        -:    3:
+        -:    4:bool on = false;
+        -:    5:int len = 42;
+        -:    6:double grid[10][10] = {0};
+        -:    7:const char * hello = "world";
+        -:    8:const char * world = "hello";
+        -:    9:
+12884901888:   10:void A::B() {}
+        -:   11:
+    #####:   12:void useless() {}
+        -:   13:
+        -:   14:double more_useless() {
+    #####:   15:  return 0;
+        -:   16:}
+        -:   17:
+        -:   18:int foo() {
+        3:   19:  on = true;
+        3:   20:  return 3;
+        -:   21:}
+        -:   22:
+        -:   23:int bar() {
+    #####:   24:  len--;
+    #####:   25:  return foo() + 45;
+        -:   26:}
+        -:   27:
+       12:   28:void assign(int ii, int jj) {
+       12:   29:  grid[ii][jj] = (ii+1) * (jj+1);
+       12:   30:}
+        -:   31:
+        -:   32:void initialize_grid() {
+       21:   33:  for (int ii = 0; ii < 2; ii++)
+       36:   34:    for (int jj = 0; jj < 2; jj++)
+       18:   35:      assign(ii, jj);
+        3:   36:}
+        -:   37:
+        -:   38:int main() {
+        3:   39:  initialize_grid();
+        -:   40:
+        3:   41:  int a = 2;
+        3:   42:  on = rand() % 2;
+        3:   43:  if (on) {
+        3:   44:    foo();
+        3:   45:    ++a;
+        3:   46:  } else {
+    #####:   47:    bar();
+    #####:   48:    a += rand();
+        -:   49:  }
+        -:   50:
+       66:   51:  for (int ii = 0; ii < 10; ++ii) {
+       30:   52:    switch (rand() % 5) {
+        -:   53:      case 0:
+        6:   54:        a += rand();
+        6:   55:        break;
+        -:   56:      case 1:
+        -:   57:      case 2:
+        3:   58:        a += rand() / rand();
+        3:   59:        break;
+        -:   60:      case 3:
+        9:   61:        a -= rand();
+        9:   62:        break;
+        -:   63:      default:
+       12:   64:        a = -1;
+       12:   65:    }
+       30:   66:  }
+        -:   67:
+        3:   68:  A thing;
+25769803782:   69:  for (uint64_t ii = 0; ii < 4294967296; ++ii)
+12884901888:   70:    thing.B();
+        -:   71:
+        3:   72:  return a + 8 + grid[2][3] + len;
+        -:   73:  return more_useless();
+        -:   74:}