Add -freroll-loops to enable loop rerolling

This adds -freroll-loops (and -fno-reroll-loops in the usual way) to enable
loop rerolling as part of the optimization pass manager. This transformation
can enable vectorization, reduce code size (or both).

Briefly, loop rerolling can transform a loop like this:

for (int i = 0; i < 3200; i += 5) {
  a[i]     += alpha * b[i];
  a[i + 1] += alpha * b[i + 1];
  a[i + 2] += alpha * b[i + 2];
  a[i + 3] += alpha * b[i + 3];
  a[i + 4] += alpha * b[i + 4];
}

into this:

for (int i = 0; i < 3200; ++i) {
  a[i] += alpha * b[i];
}

Loop rerolling is currently disabled by default at all optimization levels.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194967 91177308-0d34-0410-b5e6-96231b3b80d8
Relax some preconditions for using FixedCompilationDatabase.

FixedCompilationDatabase (FCD) requires that the arguments it consumes after
'--' must not include positional parameters or the argv[0] of the tool. This
patch relaxes those restrictions.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194968 91177308-0d34-0410-b5e6-96231b3b80d8
Remove a bad string compare from r194968

  lib/Tooling/CompilationDatabase.cpp:275:34: warning: result of comparison against a string literal is unspecified (use strncmp instead) [-Wstring-compare]

This assert() should probably be fixed and added back at some point.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194969 91177308-0d34-0410-b5e6-96231b3b80d8