[lldb] Make log for ClangModulesDeclVendor's compiler flag less verbose

Summary:
Currently the ClangModulesDeclVendor is spamming the expression log with the compiler flags it is using, which creates a log that looks like this:

```
clang

 -fmodules

 -fimplicit-module-maps
```

This patch removes all these newlines and just prints the compiler flags in one line as you see in the command line:

```
clang -fmodules -fimplicit-module-maps [...]
```

Reviewers: shafik, davide

Reviewed By: davide

Subscribers: davide, abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64858

llvm-svn: 366347